description pin assignments - diodes incorporated the al8808 is a step-down dc/dc converter designed...

AL8808 Document number: DS35648 Rev. 2 - 2

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AL8808

COST EFFECTIVE LOW EMI 30V 1A BUCK LED DRIVER

Description The AL8808 is a step-down DC/DC converter designed to drive LEDs with a constant current. The device can drive up to 8 LEDs, depending on the forward voltage of the LEDs, in series from a voltage source of 6V to 30V. Series connection of the LEDs provides identical LED currents resulting in uniform brightness and eliminating the need for ballast resistors. The AL8808 switches at frequency up to 1MHz with controlled rise and fall times to reduce EMI. This allows the use of small size external components, hence minimizing the PCB area needed. Maximum output current of AL8808 is set via an external resistor connected between the VIN and SET input pins. Dimming is achieved by applying either an analog DC voltage or a PWM signal at the CTRL input pin. An input voltage of 0.4V or lower at CTRL switches off the output MOSFET simplifying PWM dimming.

Applications • MR16 Lamps • General Illumination Lamps

Pin Assignments

Features • LED Driving Current Up to 1A • Better Than 5% Accuracy • High Efficiency Up to 96% • Fast Controlled Falling Edges 7ns • Operating Input Voltage from 6V to 30V • High Switching Frequency Up to 1MHz • PWM/DC Input for Dimming Control • Built-In Output Open-Circuit Protection • Built-In Over-Temperature Protection • TSOT25: Available in “Green” Molding Compound (No Br, Sb)

Totally Lead-Free & Fully RoHS Compliant (Notes 1 & 2) Halogen and Antimony Free. “Green” Device (Note 3)

Notes: 1. No purposely added lead. Fully EU Directive 2002/95/EC (RoHS) & 2011/65/EU (RoHS 2) compliant. 2. See http://www.diodes.com for more information about Diodes Incorporated’s definitions of Halogen and Antimony free, "Green" and Lead-Free. 3. Halogen and Antimony free "Green” products are defined as those which contain <900ppm bromine, <900ppm chlorine (<1500ppm total Br + Cl) and <1000ppm antimony compounds.

Typical Applications Circuit

TSOT25

(Top View)

1

2

3

5

4

VIN

SET

SW

GND

CTRL

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AL8808 Document number

Pin Descri

Pin Number 1 2

3

4

5

Functiona

Absolute M

Symbol ESD HBM ESD MM

VIN VSET VSW

VCTRL ISW TJ

TLEAD TST

Caution: StressefunctionaffecteSemicoand tra

r: DS35648 Rev. 2

iptions

Pin Name SW

GND

CTRL

SET

VIN

al Block Dia

Maximum

Human Body MMachine ModeContinuous VI

SET pin voltagSW voltage reCTRL pin inpuSwitch currentJunction TempLead TemperaStorage Temp

es greater than thenal operation of thed by exposure to aonductor devices a

ansporting these de

- 2

Switch Pin. CGND Pin Dimming and• Leave f

(V• Drive to• Drive w

IOUTnom

• Drive w• A PWM

current Set Nominal Input Supply section for m

agram

Ratings (@

Model ESD Proteel ESD Protectio

N pin voltage relage relative to VIN

elative to GND ut voltage t perature ature Soldering perature Range

e 'Absolute Maxime device at these obsolute maximum

are ESD sensitive aevices.

w

Connect inductor

d On/Off Control floating for normaVCTRL = 5V, Giveo voltage below 0ith an analog vol

m ith an analog vol signal (low leveto be adjusted bOutput Current PPin. Must be loc

more information.

Figure 1

TA = +25°C, unle

Parameterection n ative to GND pin

mum Ratings' specor any other conditrating conditions foand may be damag

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es nominal avera0.4V to turn off oultage (0.5V < VCT

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. AL8808 Block

ess otherwise sp

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Function ode here, minimi

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put current will beh level > 2.6; tranet by the resistor

he output current o GND with > 2.2

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cause permanent dose indicated in this of time. o ESD events. Suit

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nt IOUTnom = 0.1/R

djust output curre

e 100% of IOUTnonsition times lessr connected to Sof the device.

2µF X7R cerami

Rating2.5 200

-0.3 to -5 to +

-0.3 to -0.3 to

1.25150300

-55 to +

damage to the devis specification is n

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0 +33 0.3 +33 +6

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January 201© Diodes Incorporate

AL8808

duce EMI.

100% of

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e applications

Unit kV V V V V V A °C °C °C

tress ratings only; e reliability may be

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AL8808

Recommended Operating Conditions (@TA = +25°C, unless otherwise specified.)

Symbol Parameter Min Max Unit

VIN Operating Input Voltage Relative to GND 6.0 30 V VCTRLH Voltage High for PWM Dimming Relative to GND 2.6 5.5 V VCTRLDC Voltage Range for 20% to 100% DC Dimming Relative to GND 0.5 2.5 V VCTRLL Voltage Low for PWM Dimming Relative to GND 0 0.4 V

ISW Continuous Switch Current (Note 4) 1 A fSW Maximum Switching Frequency 1 MHz TJ Junction Temperature Range -40 +125 °C

Note: 4. Subject to ambient temperature, input voltage and switching frequency. See applications section for suggested derating.

Electrical Characteristics (@ TA = +25°C, VIN = 12V, CTRL pin floating; unless otherwise specified.)

Symbol Parameter Conditions Min Typ Max Unit VINSU Internal Regulator Start Up Voltage VIN rising 5.6 V VINSH Internal Regulator Hysteresis Threshold VIN falling 200 mV

IQ Quiescent Current VIN = 30V, output not switching 350 µA IS Input Supply Current fSW = 250kHz 450 µA

VTH Set Current Threshold Voltage 95 100 105 mV VTH-H Set Threshold Hysteresis ±20 mV ISET SET Pin Input Current VSET = VIN -0.1 16 µA

VCTRL Open Circuit CTRL Pin Voltage 5 V RCTRL CTRL Pin Input Resistance Referred to internal 5V regulator. 50 kΩ VREF Internal Reference Voltage 2.5 V

RDS(on) On Resistance of SW MOSFET ISW = 0.35A 0.35 Ω ISW_Lkg Switch Leakage Current VIN = 30V, VCTRL = 0.4V, VSENSE = 0V 0.5 µA

tR SW Rise Time VSENSE = 100 ±20mV, fSW = 250kHz VSW = 0.1V to 12V to 0.1V, CL = 15pF

7 ns

tF SW Fall Time 5 ns

TOTP Over-Temperature Shutdown

145 °C TOTP-Hyst Over-Temperature Hysteresis 10 °C

θJA Thermal Resistance Junction-to-Ambient TSOT25 (Note 5) 209 °C/W θJL Thermal Resistance Junction-to-Lead TSOT25 (Note 5) 57

θJT Thermal Resistance Junction-to-Top TSOT25 (Note 5) 13

Notes: 5. Device mounted on FR-4 PCB (25mm x 25mm 1oz copper, minimum recommended pad layout on top layer and thermal vias to maximum area bottom layer ground plane. For better thermal performance, larger copper pad for heat-sink is needed. …Refer to Figure 42 for the device derating curve.





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AL8808

Typical Performance Characteristics (@ TA = +25°C, unless otherwise stated.)

0 3 6 9 12 15 18 21 24 27 30INPUT VOLTAGE (V)

Figure 2. Supply Current vs. Input Voltage

400

350

300

250

200

150

100

50

0

INP

UT

CU

RR

EN

T (µ

A)

V = 0VV = VT = +25°C

CTRL

SET IN

A

L = 33µH T = +25°CV = 12VR = 150m1 LED

A

IN

SET

Ω

L = 68µH

L = 100µH

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VCTRL (V)

Figure 3. Switching Frequency vs. VCTRL

900

800

700

600

500

400

300

100

0

FRE

QU

EN

CY

(kH

z)

200

T = +25°CV = 12VL = 68µH

A

IN

1 LED

RSET = 100mΩ

RSET = 150mΩ

RSET = 300mΩ

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VCTRL (V)

Figure 4. LED Current vs. VCTRL

1.2

1.0

0.8

0.6

0.4

0.2

0.0

LED

CU

RR

EN

T (A

)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0VCTRL (V)

Figure 5. I vs. VCTRL CTRL

20

0

-20

-40

-60

-80

-100

-120

CTR

L P

IN C

UR

RE

NT

(µA

)T = +25°C, V = 12VL = 68µH, R

A IN

SET = 150m1 LED

Ω

0 20 40 60 80 100PWM DUTY CYCLE (%)

Figure 6. I vs. PWM Duty CycleLED

6

5

4

3

2

1

0

LED

CU

RR

EN

T E

RR

OR

(%)

0.9

LED

CU

RR

EN

T(A

)

0.6

0.3

0.0

T = +25°C, V = 12VL = 68µH, R

A IN

SET

PWM

= 150m1 LED, f = 500Hz

Ω

6 9 12 15 18 21 24 27 30INPUT VOLTAGE VCTRL (V)

Figure 7. Duty Cycle vs. Input Voltage

100

90

70

60

40

30

10

0

DU

TY C

YC

LE (%

)

80

50

20T = +25°C, R = 150mL = 33µH, L = 68µH, L = 100µH

A SET Ω


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Typical Pe

SW

MO

SFE

T R

(m)

DS

(ON

)Ω

60

50

40

30

20

10

0.50

0.45

0.40

0.35

0.30

0.25

0.20

0.15

0.00

0.05

0.10

SW

MO

SFE

T R

()

DS

(ON

)Ω

r: DS35648 Rev. 2

erformance

Figure 12. S

-40 -25 -10 5AMBI

Figure 8.

00

00

00

00

00

0

00

V = OpenV = V = 12

CTRL

SET IN

0 200

0

5

0

5

0

5

0

5

0

5

0

SWFigure 10.

V = 12VV = VT = +25°CCTRL = Floating

IN

SET IN

A

- 2

e Characte

Steady State Wa

50 6520 35ENT TEMPERAT SW R vsDS(ON)

2V

400 600

WITCH CURRENTSW R vs. SDS(ON)

w

eristics (cont

aveforms

5 80 95 110 1TURE (°C). Temperature

800 1

T (mA)Switch Current


t.) (@ TA = +25°C

25

000

m

C, unless otherw

Figu

Figur

6 9

Fig

SW

MO

SFE

T R

(m)

DS

(ON

)Ω

400

350

300

250

200

0

100

150

50V =V = VT = 25°

CTRL

SET

A

wise stated.)

ure 11. SW Outp

re 13. SW Outp

12 15 18V

ure 9. SW RIN

DS(O

= OpenV°C

IN

put Fall Time

ut Rise Time

21 248(V)

vs. Input VoltaON)


AL8808

27 30

age

3ed





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AL8808

Typical Performance Characteristics (L = 68µH, TA = +25°C, unless otherwise stated.)

6 9 12 15 18 21 24 27 30INPUT VOLTAGE (V)

Figure 14. Efficiency vs. Input Voltage

100

95

90

85

80

75

70

EFFI

CIE

NC

Y (%

)

1 LED

2 LEDs

3 LEDs4 LEDs 5 LEDs

7 LEDs

T = 25°CL= 68µHR = 150m

A

SET Ω

1 LED

2 LEDs 3 LEDs 4 LEDs 5 LEDs7 LEDs

6 9 12 15 18 21 24 27 30INPUT VOLTAGE (V)

Figure 15. 330mA LED Current vs. Input Voltage

0.36

0.35

0.34

0.33

0.32

0.31

0.30

LED

CU

RR

EN

T (A

)

T = 25°CL= 68µHR = 300m

A

SET Ω

6 9 12 15 18 21 24 27 30INPUT VOLTAGE (V)

Figure 16. Switching Frequency vs. Input Voltage

450

400

300

200

100

50

0

SW

ITC

HIN

G F

RE

QU

EN

CY

(kH

z)

350

250

1501 LED

2 LEDs 3 LEDs 4 LEDs 5 LEDs7 LEDs

6 9 12 15 18 21 24 27 30INPUT VOLTAGE (V)

Figure 17. 670mA LED Current vs. Input Voltage

0.74

0.72

0.70

0.66

0.64

0.62

0.60

LED

CU

RR

EN

T (A

)

0.68

T = 25°C, VL= 68µH, R = 150m

A

SET

IN = 12VΩ

1 LED

2 LEDs 3 LEDs

4 LEDs

5 LEDs

7 LEDs

6 9 12 15 18 21 24 27 30INPUT VOLTAGE (V)

Figure 18. Duty Cycle vs. Input Voltage

100

90

70

40

20

10

0

DU

TY C

YC

LE (%

)

80

60

30

50

1 LED

2 LEDs

3 LEDs

4 LEDs 5 LEDs 7 LEDs

T = 25°CL= 68µHR = 150m

A

SET Ω

6 9 12 15 18 21 24 27 30INPUT VOLTAGE (V)

Figure 19. 1A LED Current vs. Input Voltage

1 LED

2 LEDs

3 LEDs

4 LEDs5 LEDs

7 LEDs

T = 25°CL= 68µHR = 100m

A

SET Ω

1.10

1.05

0.95

0.90

LED

CU

RR

ENT

(A)

1.00





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AL8808

Typical Performance Characteristics (670mA LED Current; TA = +25°C unless otherwise stated.)

186 9 12 15 21 24 27 30INPUT VOLTAGE (V)

Figure 20. LED Current Deviation vs. Input Voltage

DE

VIA

TIO

N F

RO

M T

AR

GE

T C

UR

RE

NT

(%) 5

3

-3

-5

1

-1

4 LEDs

5 LEDs

7 LEDs

1 LED

2 LEDs 3 LEDs

T = 25°C,L = 33µHR = 150m

A

SET Ω

186 9 12 15 21 24 27 30INPUT VOLTAGE (V)


SW

ITC

HIN

G F

RE

QU

EN

CY

(kH

z)

500

400

100

0

300

200

800

700

600

4 LEDs 5 LEDs7 LEDs

1 LED

2 LEDs 3 LEDs

T = 25°C,L = 33µHR = 150m

A

SET Ω

186 9 12 15 21 24 27 30INPUT VOLTAGE (V)


DE

VIA

TIO

N F

RO

M T

AR

GE

T (%

)

2

1

-2

-5

0

-1

5

4

3

-4

-3

4 LEDs5 LEDs

7 LEDs

1 LED

2 LEDs3 LEDs

T = 25°C,L = 68µHR = 150m

A

SET Ω

186 9 12 15 21 24 27 30INPUT VOLTAGE (V)


SW

ITC

HIN

G F

RE

QU

EN

CY

(kH

z)

300

250

50

0

200

150

450

400

350

100

T = 25°C,L = 100µHR = 150m

A

SET Ω

4 LEDs 5 LEDs

7 LEDs

1 LED

2 LEDs3 LEDs

186 9 12 15 21 24 27 30INPUT VOLTAGE (V)


DE

VIA

TIO

N F

RO

M T

AR

GET

(%)

2

1

-2

-5

0

-1

5

4

3

-4

-3

186 9 12 15 21 24 27 30INPUT VOLTAGE (V)


SW

ITC

HIN

G F

RE

QU

EN

CY

(kH

z)

150

0

100

250

200

50





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AL8808

Typical Performance Characteristics (1A LED Current) TA = +25°C unless otherwise stated.)

1 LED

2 LEDs3 LEDs 4 LEDs 5 LEDs 7 LEDs

T = 25°CL = 33µHR = 100m

A

SET Ω

1815126 9 30272421INPUT VOLTAGE (V)


5

4

3

2

-4

-5

0

DE

VIA

TIO

N F

RO

M T

AR

GET

CU

RR

ENT

(%)

1

-1

-2

-3

1815126 9 30272421INPUT VOLTAGE (V)


600

500

400

300

100

0

200

SWIT

CH

ING

FR

EQ

UE

NC

Y (k

Hz)

1 LED

2 LEDs 3 LEDs 4 LEDs 5 LEDs 7 LEDs

T = 25°CL = 33µHR = 100m

A

SET Ω

1815126 9 30272421INPUT VOLTAGE (V)


1 LED

2 LEDs

3 LEDs4 LEDs

5 LEDs7 LEDs

T = 25°CL = 68µHR = 100m

A

SET Ω

5

4

3

2

-4

-5

0

DE

VIAT

ION

FR

OM

TAR

GE

T C

UR

REN

T (%

)

1

-1

-2

-3

1815126 9 30272421INPUT VOLTAGE (V)


300

250

200

150

50

0

100

SW

ITC

HIN

G F

RE

QU

ENC

Y (k

Hz)

1815126 9 30272421INPUT VOLTAGE (V)


1 LED

2 LEDs 3 LEDs4 LEDs

5 LEDs 7 LEDs

T = 25°CL = 100µHR = 100m

A

SET Ω

5

4

3

2

-4

-5

0

DE

VIA

TIO

N F

RO

M T

AR

GE

T C

UR

RE

NT

(%)

1

-1

-2

-3

1815126 9 30272421INPUT VOLTAGE (V)


200

180

160

80

20

0

60

SW

ITC

HIN

G F

RE

QU

EN

CY

(kH

z)

140

100

120

40

1 LED


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Applicatio AL8808 OperaIn normal operainductor L1, andvoltage and the

This rising currevoltage to the in

When this voltagL1, the LEDs anof the LEDs and

This decaying capplied at the inThis switch-on-a

LED Current CThe LED currenconnected from with CTRL left flo

If the CTRL pin i

For example for

When the CTRL

r: DS35648 Rev. 2

on Informat

ation ation, when voltad the LEDs. Theinductor L1.

ent produces a voput of the interna

ge reaches an innd the schottky dd the schottky dio

urrent produces nput of the internand-off cycle con

Control nt is controlled b

the CTRL pin tooating. If the CT

is driven by an e

a desired LED c

L voltage is broug

- 2

tion

age is applied at e current ramps

oltage ramp acroal comparator.

nternally set uppdiode D1, and baode.

a falling voltageal comparator. Wtinues to provide

by the resistor Ro an internal 5V rRL pin is left floa

xternal voltage (

current of 660mA

ght below 0.4V, t

w

+VIN, the AL880up linearly, and

oss R1. The inter

per threshold, theack to the supply

e at R1, which is When this voltage the average LE

Figure 32.

1 (in Figure 32) regulator. When ating or driven ab

Where

higher than 0.5V

IL

Where

A and VCTRL=2.5V

=RSET

he output switch


08 internal switcd the ramp rate

rnal circuit of the

e internal switch y rail, but it decay

sensed by the Ae falls to the inte

ED current set by

Typical Applica

connected betwthe CTRL pin is

bove 2.5V the no

1RVI TH

LED =

VTH is nominally

V and lower than

RV

VV T

REF

CTRLLED =

e VREF is nomina

V or with the CTR

≈==66.01.0

IV

LED

TH

h is turned off wh

m

ch is turned on. is determined b

AL8808 senses

is turned off. Thys, with the rate

AL8808. A voltagernally set lower

y the sense resist

ation Circuit

ween VIN and SEs left floating it geominal average o

y 100mV.

2.5V), the avera

1RTH

ally 2.5V

RL pin left open

Ωm150

ich allows PWM

Current starts toby the input volta

s the voltage acro

he inductor curreof decay determ

ge proportional tor threshold, the intor R1.

ET pins. The AL8ets pulled up to 5utput current in t

age LED current

the resulting res

dimming.

o flow through seage (+VIN) minu

oss R1 and appli

ent continues to mined by the forw

o the sense voltnternal switch is

8808 has an inte5V - increasing ithe LED(s) is def

is:

istor is:


AL8808

ense resistor R1us the LED chain

ies a proportiona

flow through R1ward voltage drop

age across R1 isturned on again

ernal 50k resistoits noise rejectiofined as:

3ed

, n

al

, p

s n.

or n





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AL8808

Application Information (cont.) Analog Dimming The CTRL pin can be driven by an external analog voltage (VCTRL), to adjust the output current to a value below the nominal average value defined by R1. The LED current decreases linearly with the CTRL voltage when 0.5V ≤ VCTRL ≤ 2.5V.

Note that 100% brightness is achieved when either the CTRL pin is left floating or pulled above 2.5V by an external voltage source.

For 2.6V ≤ VCTRL ≤ 5.5V the LED current will not get overdriven and will be set the current according to the equation VCTRL = 2.5V (the internal reference voltage). See Figure 33 below.

PWM Dimming LED current can be adjusted digitally, by applying a low frequency Pulse Width Modulated (PWM) logic signal to the CTRL pin to turn the device on and off. This will produce an average output current proportional to the duty cycle of the control signal. In particular, a PWM signal with a max resolution of 10bit (~0.1% duty cycle) can be applied to the CTRL pin to change the output current to a value below the nominal average value set by resistor RSET. To achieve this resolution the PWM frequency has to be lower than 500Hz, however higher dimming frequencies can be used, at the expense of dimming dynamic range and accuracy. Typically, for a PWM frequency of 500Hz the accuracy is better than 2% for PWM ranging from 5% to 100%.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0V (V)

Figure 33. LED Current Dimming Ration andTypical Error vs. Control Voltage

CTRL

110

100

90

80

70

60

50

40

30

20

10

0

DIM

ME

D :

100%

LE

D C

UR

RE

NT

RAT

IO (%

)

LED Current Error

Dimmed LED Current Ratio2.0

2.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

TYP

ICA

LE

RR

OR

(%)

T = 25°CV = 12VL = 68µH

A

IN

50 600 10 20 30 40 90 10070 80PWM DUTY CYCLE (%)

Figure 34. PWM Dimming at 500Hz

0.8

0.7

0.1

0

0.3

0.2

0.6

0.5

0.4

LED

CU

RR

EN

T (A

)

V = 12VT = +25°CL = 68µHR = 150m1 LED

IN

A

SET Ω

V = 12VT = +25°CL = 68µHR = 150m1 LED

IN

A

SET Ω

7.50.0 2.5 5.0 15.010.0 12.5PWM DUTY CYCLE (%)

Figure 35. Low Duty Cycle PWM Dimming at 500Hz

LOAD

CU

RR

EN

T E

RR

OR

(%)

10

9

8

7

6

5

4

3

2

1

10


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Applicatio PWM DimminThe CTRL pin isoutput stage. The ultimate PWlower switching the accuracy/line At the start of ethreshold isn’t re

Greater PWM dfrequency. The three figurefrom a 12V rail a

Figure 36. 0

Figure 38. 0

As can be obsevalue.

r: DS35648 Rev. 2

on Informat

ng (cont.) s designed to be

WM dimming resofrequencies andearity of the PWM

each PWM cycleeached then the

dimming dynami

s below show 0.at +25°C for a no

.2% PWM Duty 68µH

.2% PWM Duty 22µH

rved from Figure

- 2

tion (cont.)

e driven by both

olution is determ/or higher PWM M dimming.

e the LED curreaccuracy will be

c ranges can be

2% duty cycle Pominal LED curre

Cycle at 100Hz H Inductance

Cycle at 500Hz H Inductance

e 37 greater dim

w

3.3V and 5V log

ined by the numfrequencies the

ent needs to resgreatly affected.

e achieved by r

PWM pulse resolent of 670mA.

PWM Frequenc

PWM Frequenc

mming accuracy c


gic levels directly

ber of full LED snumber of full sw

start from zero u. reducing the PW

ution with differe

cy and Fig

cy and

can be achieved

m

y from a logic ou

switching cycles twitching cycles t

up to the upper

WM dimming fre

ent PWM frequen

gure 37. 0.2% P

d by reducing bo

tput with either a

that can be achiethat can be achie

r threshold level

quency and/or i

ncies and differe

WM Duty Cycle22µH Indu

th the PWM dim

an open drain ou

eved during the eved is reduced

(nominally 120

ncreasing the A

nt inductor value

e at 100Hz PWMuctance

mming frequency


AL8808

utput or push pu

PWM on-time. Athereby reducing

mV/RSET). If this

AL8808 switching

es driving 2 LEDs

Frequency and

and the inducto

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ll

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s

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s

d

or


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Applicatio Start-up and SOn initial power than 0.45V (typithe output starts

This will cause sthe upper LED c The AL8808 doeaccuracy.

Soft start can berail on the CTRL

The external capoccurs it is esse

This is achievedthe control volta Adding this capadimming accura

Reducing OutPeak to peak ripA value of 1μF capacitor values Note that the cavoltage. By addaltering the mea

r: DS35648 Rev. 2

on Informat

Soft Start up the device w

cal). This causess switching the LE

some additional icurrent threshold

es not have in-bu

e easily implemeL pin will charge t

pacitor slows upntial that the cap

d by increasing thge at the input of

acitor increases cy depending on

Figure

tput Ripple pple current in the

will reduce the s.

apacitor will not ing this capacito

an current value.

- 2

tion (cont.)

will not start switcs a slight delay (ED current will b

nput current to thlevel by slowing

uilt soft-start acti

ented by adding athe external capa

the ramp-up of pacitor is large en

he time taken forf the comparator

the time taken fn the delay introd

39. Soft Start w

e LED(s) can be supply ripple c

affect operatingor the current wa

w

ching until the p(dependent on rauild up to the upp

ILEDS

hat of charging thg down the rise o

ion allowing very

an external capaacitor up to 5V.

the CTRL pin vonough to keep th

r the CTRL voltar.

for the output toduced.

with 100nF Cap

reduced, if requurrent significan

g frequency or eaveform through


ower supply hasamp rate of inpuper threshold lev

1.0V5.2

VCTRLSSPK =

he input bulk capf the CTRL pin v

y fast control of t

acitor from the C

oltage thereby rehe CTRL pin volta

age to rise to the

o reach 90% of i

pacitor on CTRL

ired, by shuntingntly in the typica

efficiency, but it wthe LED(s) chan

m

s reached approxt voltage and inp

vel:

1R2.11×

pacitance. One wvoltage – implem

the output Power

CTRL pin to groun

educing the LEDage below 2.5V d

upper (turn-off)

ts final value, th

L pin (VIN = 24V,

g a capacitor C2 al case. Proportio

will increase stanges from a trian

ximately 5.6V orput bulk capacita

way of reducing tenting a soft-sta

r MOSFET switc

nd. The internal

current via analduring the ramp-

threshold and by

his delay is 25µs

ILED = 667mA, 1

across the LED(onally lower ripp

art-up delay, by ngular ramp to a

r the CTRL pin vance of the AL88

this additional curt.

ch which improve

pull-up resistor t

log dimming. To -up of the input v

y slowing down t

s/nF, but will imp

1 LED)

(s) as shown in Fple can be achie

reducing the rata more sinusoida


AL8808

voltage is greate808 circuit). Once

urrent is to reduce

es PWM dimming

to the internal 5V

ensure soft-starvoltage.

the rate of rise o

pact on the PWM

Figure 32. eved with highe

te of rise of LEDal version withou

3ed

er e

e

g

V

rt

of

M

er

D ut


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Applicatio Inductor SeleRecommended data for selecterequired switchiLED current acc

The inductor shoThe chosen coil required mean o

Suitable coils for

Part

MSS1038-333

MSS1038-683

NPIS64D330M The inductor valThe following eq Switch ‘On’ tim

V

tIN

ON −=

Switch ‘Off’ tim

V

tL

OFF =

Where:

L is the coil rL is the coil RS is the curIavg is the reVIN is the suΔI is the coilVLED is the tRSW is the sVD is the dio

r: DS35648 Rev. 2

on Informat

ection inductor values

ed component vang frequency. L

curacy (due to pro

ould be mountedshould have a s

output current.

r use with the AL

No.

MTRF

ue should be choquations can be u

me:

xIVL

AVGLED −−Δ

me:

IVIL

AVGDLED ++Δ

inductance (H) resistance (Ω) rrent sense resisquired LED curre

upply voltage (V) l peak-peak rippltotal LED forwardswitch resistanceode forward volta

- 2

tion (cont.)

for the AL8808 alues. The inducLower inductor vopagation delays

Figure 4

d as close to the dsaturation current

L8807 are listed iL

(µH) D(

33 0.

68 0.

33 0.

osen to maintainused as a guide,

( RrRxI

SWLS ++

( )rRx LSG +

stance (Ω) ent (A)

e current (A) Intd voltage (V) (Ω) =0.35Ω nom

age at the require

w

are in the rangectance used willalues can be uss) and increase p

0. Inductor Valu

device as possibt higher than the

in the table belowDCR (V)

ISAT(A)

093 2.3

213 1.5

124 1.1

n operating duty c with reference t

)W

ternally set to 0.4

minal ed load current (V


e 33µH to 100µH depend on a c

sed to increase tpower dissipation

ue with Input Volt

ble with low resispeak output cur

w: T )

3 Co

5

1

cycle and switcho Figure 1 - Ope

4 x IAVG

V)

m

H. Note that thecombination of Inhe switching freqn (due to switchin

tage and Numbe

stance/stray inducrrent and a contin

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AL8808

Application Information (cont.) Capacitor Selection The small size of ceramic capacitors makes them ideal for AL8808 applications. X7R type is recommended because it retains capacitance value over wider voltage and temperature ranges than other types such as Y5V or Z5U. X5R is a useful compromise over a restricted temperature range. Note that even X7R capacitance reduces significantly with increased DC bias voltage. At 50% of rated voltage, the capacitance loss is between about 10% to 50% of nominal. Therefore it is often necessary to select a voltage rating which is at least twice the operating voltage. Input Capacitor In Figure 32, the input capacitor C1 is required as a reservoir. Diode D1 switches at a rate of typically up to 400kHz. The power supply has a finite impedance, often including a wiring inductance value of the order of 100nH to 1uH or more depending upon the system design. C1 is required to limit the power supply voltage and current ripple both to allow stable regulation of the LED current, and also to meet EMC requirements. A 2.2μF input capacitor is sufficient for most DC powered applications of AL8808. This depends upon the operating voltage and current and the maximum level of ripple required. Additional capacitors may be required in parallel for EMC purposes. This is described below in a separate section. However, if operated from a rectified low voltage AC source, such as MR16, then the input capacitance will need to be significantly increased to provide enough reservoir charge when the input voltage falls below the minimum operating voltage of the AL8808 or the LED chain voltage Output Capacitor In Figure 32, the output capacitor C2 is normally required to limit the load voltage and current ripple, in order to meet EMC requirements. A value of 0.1µF to 1µF is sufficient for many requirements, depending on voltage and current conditions. Additional capacitors may be required in parallel for EMC purposes. This is described below in a separate section. Diode Selection For maximum efficiency and performance, the flywheel rectifier (D1) should be a fast low capacitance Schottky diode with low reverse leakage at the maximum operating voltage and temperature. The silicon PN diode is not suitable because of its increased power loss, due to a combination of lower forward voltage and reduced recovery time. The use of a Super-Barrier-Rectifier (SBR) is not recommended for use as a flywheel diode in this application. (However the SBR provides significant advantages when used with an AC power input as a bridge rectifier driving VIN.)

It is important to select D1 with a peak current rating above the peak coil current and a continuous current rating higher than the maximum output load current. In particular, it is recommended to have a diode voltage rating at least 15% higher than VIN to ensure safe operation during the switching and a mean current rating at least 10% higher than the peak diode current. The power rating is verified by calculating the power loss through the diode. In practice, the voltage rating selection is often increased by up to about 50% to obtain a better compromise with loss due to reverse leakage current at higher temperature. Also the current rating is typically selected to provide a margin of up to about 50%. Schottky diodes, e.g. DFLS240L or DFLS140, with their low forward voltage drop and fast reverse recovery, are the ideal choice for AL8808 applications. Leakage current is sufficiently limited for the application.





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AL8808

Application Information (cont.) Thermal Considerations For continuous conduction mode of operation, the absolute maximum junction temperature must not be exceeded. The maximum power dissipation depends on several factors: the thermal resistance of the IC package (θJC), PCB layout, airflow surrounding the IC, and difference between junction and ambient temperature. The maximum power dissipation can be calculated using the following formula:

JA

A)MAX(J)MAX(D

TTP

θ−

=

where TJ(MAX) is the maximum operating junction temperature; for the AL8808 this is +125°C. TA is the ambient temperature, and θJA is the junction to ambient thermal resistance. The major thermal path for the TSOT25 package is pin 2 (GND pin) and it is important for minimizing the θJA that a suitable area and thermal mass is associated with pin 2. The thermal impedance from the AL8808 junction to pin 2 is approximately 57°C/W. The AL8808’s θJA on a 25 x 25mm double sided FR4 PCB with minimum recommended pad layout on top layer and thermal vias to maximum area on bottom layer with 1oz copper standing in still air is approximately 209°C/W. Yielding a maximum power dissipation at 25°C of 0.47W The AL8808’s θJA on a 25 x 25mm double sided FR4 PCB with maximum area top and bottom with vias is approximately 151°C/W; which gives a maximum power dissipation at 25°C of 0. 66W. Figure 42 shows the power derating of the AL8808 on different area PCB with maximum area on bottom of PCB with 1 and 2oz copper standing in still air.

Figure 42. Derating Curve for Different PCB

(50mm) with 2oz Cu max area top2



(25mm) with 1oz Cu min area top2




-10 5-40 -25 50 6520 35 110 12580 95AMBIENT TEMPERATURE (C)

PO

WE

R D

ISS

IPAT

ION

(W)

1.0

0.9

0.1

0

0.8

0.7

0.3

0.2

0.6

0.5

0.4


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Applicatio EMI and LayoThe AL8808 is decoupling and switching speeddue to switching The turn-on edgand PCB tracksbetween the Sch The tracks fromshort as possibleThere is an induestimate the primand length will d The resonant frediode. An exam

Summary: 1. Use a

the gr(EMC

2. Place less ththe inparasimore g

3. Place 4. Place 5. Place

capacimped

r: DS35648 Rev. 2

on Informat

out Considerata switching reglayout of the PC

ds of the internalg losses and radia

ge (falling edge) . After the Schohottky diode capa

the SW pin to te.

uctance internallymary resonant frdominate the size

equency of any mple of good layo

Figure 43. PC

PCB constructioround areas are ) operation and acapacitor C1 as

han about 5mm. put current passitic inductance ofground via holessense resistor RD1 anode, the Scapacitor C2 as

citor and its coppdance coupling d

- 2

tion (cont.)

tions ulator with fast

CB.To help with power MOSFETated EMI.

dominates the raottky diode reversacitance and the

the Anode of the

y in the AL8808 tequency. If the t

e of the inductanc

oscillation is detut is shown in Fi

CB Loop Reson

on with copper fotightly connecte

also to minimize close as possib To ensure the b

ses directly throuf the copper trac

s close to the groR1 as close as poSW pin and the ins close as poss

per trace such thaue to the added

w

edges and meathese effects theT. The rise and

adiated EMI whise recovery timee track inductanc

e Schottky diode

this can be assurack is capable oce.

termined by the gure 44 - the str

ance

oil on top and boed together usingdevice temperatle to VIN, and as best possible EMugh the capacitoce. Ensure low inound pad. ossible to VIN andnductor as closeible to L1 and Sat the input curreparasitic inducta


asures small diffe AL8808 has b

d fall times are c

ch is due to an i of around 5ns he, LTRACK, See F

e, D1, and then f

med to be arounof handling 1A in

combined inducay track inductan

ottom. Provide mg plated via holetures by spreadinclose as possib

MI filtering (greater mounting pad. nductance conne

d SET. together as poss

SET. To ensureent passes direc

ance of the coppe

m

ferential voltageseen developed t

controlled to get

interaction betwehas occurred; theFigure 43.

from D1’s cathod

nd 1nH. For PCBncreasing the thic

ctance in the trance should be le

Figure

maximum coverages placed at regng the dissipatedle to the cathodeest attenuation),

This minimizesection between th

sible to avoid rine the best possibctly through the cer trace.

s; as a result ofto minimise radiathe right compro

een the Schottkye falling edge of

de to the decoup

B tracks a figure ckness will have

ck and the effecss than 5nH.

44. Recommen

ge of copper groular intervals. T

d heat. e of D1. The sepplace the capaci

s common impedhe capacitor and

ging. ble EMI filtering capacitor mountin

f this care has ated emissions bomise between p

y diode (D1), Swthe SW pin sees

pling capacitors

of 0.5nH per mme a minor effect o

ctive capacitance

nded PCB Layo

und plane on boThis is required

paration of these itor and its coppedance coupling dd its ground conn

(greatest attenung pad. This mi


AL8808

to be taken withby controlling thepower dissipation

witching MOSFETs a resonant loop

C1 should be as

m can be used toon the inductance

e of the Schottky

ut

oth sides. Ensureboth for low EM

nodes should beer trace such thadue to the addednection. Use 2 o

uation), place thenimizes common

3ed

h e n

T p

s

o e

y

e MI

e at d

or

e n





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AL8808

Application Information (cont.) EMI and Layout Considerations (cont.) EMC Design In addition to the layout instructions above, it may be necessary to take further measures to reduce electromagnetic interference (EMI) and meet EMC requirements. This depends on the speed of the switching transitions. The fast switching edges include spectral harmonics spreading into the UHF frequency range towards 500MHz. In this respect, AL8808 has been optimized to shape the switching current waveform to minimize EMI while maintaining fast enough switching for high power efficiency. However, depending on the physical system design it may be necessary to add additional filtering to reduce radiated and conducted emissions. The required circuit changes depend on a number of system design aspects including the PCB size, the housing design and the length of external connecting wires. Radiated Emission Typically, the filtering required to control radiated emission consists of one or two additional capacitors placed close to the connecting points of the wires. Very often the frequency range requiring most attenuation is in the region of 100MHz to 500MHz. In order to provide best attenuation in this frequency range, use a capacitor of 1000pF to 2200pF with COG dielectric type, rated 50V or 100V. This capacitor provides very low ESR in this frequency range. Place two such capacitors, one near the VIN wire connection and one near the output connection to L1. Again, to ensure the best possible EMI filtering (greatest attenuation), place the capacitor and its copper trace such that the input or output current passes directly through the capacitor mounting pad. This minimizes common impedance coupling due to the added parasitic inductance of the copper trace. Conducted Emission Conducted emission limits sometimes require filtering in the lower frequency range, from the switching frequency itself (Typically 200kHz) up to about 30MHz. Usually the requirement only applies on the input side. The existing power supply may already include suitable measures. If necessary add an input capacitor to reduce the ripple in this frequency range. Again the capacitors and their copper traces should be carefully placed to avoid inductive common impedance coupling. Sometimes an additional series filter inductor may be added to achieve the desired attenuation. An additional shunt capacitor to ground is connected resulting in a pi-filter configuration.


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Applicatio Fault ConditioThe AL8808 hasVIN and the SW If the LEDs shouthe expected cudramatically andLEDs. The on-time of tinductor. The off32) causing a m

High TemperaThe AL8808 is atemperature in temperatures anAL8808 off. This The OTP shutdonever switch-off junction tempera

r: DS35648 Rev. 2

on Informat

on Operation s by default openpin will then fall

uld become shorurrent - so no ed the switching f

the internal powf-time is significauch slower deca

ature Operatioa high efficiency excess of 100°

nd hence junctios will allow the ju

own junction temwith a junction t

ature of the AL88

- 2

tion (cont.)

n LED protectionto GND. No exce

rted together theexcessive heat wrequency will mo

er MOSFET swiantly increased bay in inductor cur

Figure 45. Sw

on and Protecswitching LED dC given the co

on temperature tnction temperatu

mperature of thetemperature belo808.

w

n. If the LEDs shessive voltages w

AL8808 will conwill be generatedost likely decrea

itch is significantecause the reve

rrent.

witching Charac

tion river capable of

orrect thermal imthen the Over-Teure of the AL880

e AL8808 is appow +125°C allow


hould become opwill be seen by th

ntinue to switch ad within the AL8se. See Figure 4

tly reduced becarse voltage acros

cteristics (norma

operating junctiompedance to freemperature Prot8 to cool down a

roximately +145wing the designe

m

pen circuit the ALhe AL8808.

and the current t8808. However,45 for an examp

ause almost all oss the inductor is

al open to short

on temperatures ee air. If a fault tection (OTP) of and potentially giv

°C with a hysterr to design the s

L8808 will stop o

hrough the AL88the duty cycle

ple of this behav

of the input voltas now just the Sc

t LED chain)

up to +125°C. Tshould occur t

f the AL8808 wilving an opportun

resis of +10°C. system thermally

oscillating; the SE

808’s internal swat which it operior at 24V input

age is now devechottky diode vol

his allows it opethat leads to incll cut in turning tnity for the fault to

This means thatto fully utilize th


AL8808

ET pin will rise to

witch will still be arates will changevoltage driving 3

eloped across theltage (See Figure

rate with ambiencreased ambienthe output of theo clear itself.

t the AL8808 wihe wide operating

3ed

o

at e 3

e e

nt nt e

ll g





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AL8808

Ordering Information

Part Number Package Code Packaging 7” Tape and Reel

Quantity Part Number Suffix AL8808WT-7 WT TSOT25 3000/Tape & Reel -7

Marking Information

TSOT25

1 2 3

5 74

XX Y W X

(Top View)

XX : Identification code

W : Week : A~Z : 1~26 week;

X : A~Z : Internal code

Y : Year 0~9

a~z : 27~52 week; z represents52 and 53 week

Part Number Package Identification Code AL8808WT-7 TSOT25 B9

Package Outline Dimensions (All dimensions in mm.) Please see AP02002 at http://www.diodes.com/datasheets/ap02002.pdf for latest version.

AL8808 XX XX-

WT : TSOT25 7 : 7” Tape & Reel

PackingPackage

TSOT25 Dim Min Max Typ

A − 1.00 −A1 0.01 0.10 − A2 0.84 0.90 − D − − 2.90E − − 2.80

E1 − − 1.60b 0.30 0.45 −c 0.12 0.20 −e − − 0.95

e1 − − 1.90L 0.30 0.50

L2 − − 0.25θ 0° 8° 4° θ1 4° 12° −All Dimensions in mm

c

A1

L

E1 E

A2

De1

e5x b

θ

4x 1 θ

L2

A


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AL8808

Suggested Pad Layout Please see AP02001 at http://www.diodes.com/datasheets/ap02001.pdf for latest version.

IMPORTANT NOTICE DIODES INCORPORATED MAKES NO WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, WITH REGARDS TO THIS DOCUMENT, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE (AND THEIR EQUIVALENTS UNDER THE LAWS OF ANY JURISDICTION). Diodes Incorporated and its subsidiaries reserve the right to make modifications, enhancements, improvements, corrections or other changes without further notice to this document and any product described herein. Diodes Incorporated does not assume any liability arising out of the application or use of this document or any product described herein; neither does Diodes Incorporated convey any license under its patent or trademark rights, nor the rights of others. Any Customer or user of this document or products described herein in such applications shall assume all risks of such use and will agree to hold Diodes Incorporated and all the companies whose products are represented on Diodes Incorporated website, harmless against all damages. Diodes Incorporated does not warrant or accept any liability whatsoever in respect of any products purchased through unauthorized sales channel. Should Customers purchase or use Diodes Incorporated products for any unintended or unauthorized application, Customers shall indemnify and hold Diodes Incorporated and its representatives harmless against all claims, damages, expenses, and attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such unintended or unauthorized application. Products described herein may be covered by one or more United States, international or foreign patents pending. Product names and markings noted herein may also be covered by one or more United States, international or foreign trademarks. This document is written in English but may be translated into multiple languages for reference. Only the English version of this document is the final and determinative format released by Diodes Incorporated.

LIFE SUPPORT Diodes Incorporated products are specifically not authorized for use as critical components in life support devices or systems without the express written approval of the Chief Executive Officer of Diodes Incorporated. As used herein: A. Life support devices or systems are devices or systems which: 1. are intended to implant into the body, or

2. support or sustain life and whose failure to perform when properly used in accordance with instructions for use provided in the labeling can be reasonably expected to result in significant injury to the user.

B. A critical component is any component in a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or to affect its safety or effectiveness. Customers represent that they have all necessary expertise in the safety and regulatory ramifications of their life support devices or systems, and acknowledge and agree that they are solely responsible for all legal, regulatory and safety-related requirements concerning their products and any use of Diodes Incorporated products in such safety-critical, life support devices or systems, notwithstanding any devices- or systems-related information or support that may be provided by Diodes Incorporated. Further, Customers must fully indemnify Diodes Incorporated and its representatives against any damages arising out of the use of Diodes Incorporated products in such safety-critical, life support devices or systems. Copyright © 2013, Diodes Incorporated www.diodes.com

Dimensions Value (in mm) C 0.950 X 0.700 Y 1.000 Y1 3.199 Y1

C C

X (5x)

Y (5x)


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description pin assignments - diodes incorporated the al8808 is a step-down dc/dc converter designed...

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