an3263, external soft start and dimming control for dc-dc...

AN3263External Soft Start and Dimming Control

for DC-DC Boost LED Driver

INTRODUCTION

The proliferation of portable electronic devices withLiquid Crystal Displays (LCDs), such as smart phones,handheld games and portable computers, has increasedthe demand for using white LEDs as a backlight sourcefor the LCDs. The white LEDs are used as a light sourcebecause of their high efficiency, small size, lower costand long operating life. Series string clusters of LEDs areusually used for higher brightness and better brightnessmatching between LEDs; therefore, a high-efficiencyDC-DC boost converter is used to drive the LEDs inseries for the LCD backlight in these battery-poweredhandheld electronic devices. In this application note,some external soft start and dimming control circuits forsimple DC-DC boost LED drivers will be discussed.

SOFT START CIRCUIT

In DC-DC boost converters, the inrush current isapproximately double, or more than the nominal inputcurrent level during start-up, to set up the outputvoltage on the output capacitor and LED string load.High inrush current will drain excessive power from thebattery, cause output voltage overshoot, generatemore heat dissipation and power loss, which reducesthe battery usable operating time. To reduce the inrushcurrent, prevent output voltage overshoot and save thebattery power, a soft start circuit should be used to limitthe inrush current. For some simple DC-DC boostconverters, which may not have sufficient soft start or alonger soft start time is required, the soft start circuit inFigure 1 can be implemented.

FIGURE 1: MIC2289 Boost Converter LED Driver Application Circuit with External Soft Start.

Author: Indy Ho, Surya TalariMicrochip Technology Inc.

VIN

2200 pF

VIN

EN

SW

OUT

FBGND

D1D2

CSS

(Optional)

RSS 10k

VOUT

2019 Microchip Technology Inc. DS00003263A-page 1

AN3263

The circuit uses a soft start capacitor, CSS, to controlthe output voltage rise time by providing extra feedbackfrom the output to the FB pin. The output voltage startsto rise when the boost converter starts switching. ThedV/dt of the output voltage forces a current through thesoft start capacitor CSS and lower resistor RSS. Thisaction provides an overdrive voltage to the FB pinthrough the diode D1 during start-up. This increasedvoltage on the FB pin reduces the switching duty cycleof the boost converter, limiting the turn-on time of theboost switch and the inductor switching current. As theCSS capacitor is charging up, the overdrive voltage onthe FB pin is slowly reducing, and this results in aninitial reduced switching duty cycle and then gradualincrease of the switching duty cycle, up to the finalnominal value in regulation steady state, and limitedinrush current and slow ramping up of output voltage.

Increasing the value of the CSS capacitor causes theoutput voltage to rise more slowly and increases thesoft start time. The increased CSS capacitor valuecauses the CSS capacitor to charge up more slowly andthe overdrive voltage on the FB pin to decay moreslowly. This results in a more gradual increase ofswitching duty cycle, up to nominal value and longeroutput voltage ramp-up time. The lower resistor, RSS,together with CSS, controls the RC decay time of theoverdrive voltage seen by the FB pin and RSS ensuresthe D1 is reverse-biased after soft start. It also providesa discharge path for CSS when the boost converterturns off. The value of RSS should be in the range ofkOhm for most applications, while the CSS should be inthe range of a few nF to a few µF.

Diode D1 provides a conduction path for the overdrivevoltage to the FB pin during soft start. D1 isreverse-biased in normal operation after soft starting theoutput and prevents CSS from appearing in parallel withnormal feedback, which would affect the stability andtransient response. A signal diode should be used for theD1 since a signal diode has very small reverse leakagecurrent. Using a Schottky diode is not recommended forD1, as it would have larger reverse leakage current whichwill affect the normal feedback voltage signal after softstart. The optional Zener diode, D2, clamps the voltageseen by the FB pin below the FB pin’s maximum ratingand provides a discharge path for CSS when the boostconverter is turned off. The soft start time can beapproximated by the following equation:

EQUATION 1:

Design Example 1

Let’s consider a practical application for driving sixwhite LEDs with the MIC2289 supplied by a single-cellLi-ion battery.

The design parameters are given as below:

VOUT = N × VF + VREF

VOUT = 6 × 3.6V + 0.095V

VOUT = 21.695V

10 kΩ is selected as RSS for convenience.

The value of CSS can be calculated from Equation 1 asfollows:

EQUATION 2:

Therefore, 2200 pF is chosen for CSS to ensure thetarget soft start time can be achieved.

Input Voltage (VIN): 3.6V

Output: Six LEDs in Series

Reference (VREF): 95 mV

Forward Voltage (VF): 3.6V (single LED)

Number of LEDs (N): 6

Soft Start Time (tSS): 200 µs

tSS 2 CSS RSS

VOUT

VREF------------- ln

Where:

CSS = External Soft Start CapacitorRSS = Resistor of the Soft Start CircuitVOUT = Output VoltageVREF = Internal Reference Voltage

CSS

tSS

2 RSS

VOUT

VREF------------- ln

-------------------------------------------------

CSS200s

2 10k 21.695V0.095V-------------------- ln

-------------------------------------------------------------

CSS 1.84nF

DS00003263A-page 2 2019 Microchip Technology Inc.

AN3263

Figure 2 shows the turn-on of the MIC2289 boost LEDdriver, driving six white LEDs in series, without theexternal soft start circuit. Figure 3 shows the turn-on ofthe same LED driver with the external soft start circuitusing CSS = 2200 pF and RSS = 10 kΩ. It shows thatthe soft start circuit has reduced the inrush current andprevents output voltage overshoot.

FIGURE 2: 6-Series LED Circuit without External Soft Start.

FIGURE 3: 6-Series LED Circuit with External Soft Start.

DIMMING CONTROL

Adjusting LED brightness is usually required in theactual end product application, such as increasing thebrightness of the LCD in brighter environments toimprove the readability of the display content. Thereare two types of dimming methods: one method isPWM dimming and another one is continuous analogdimming.

PWM Dimming

PWM dimming control is implemented by applying aPWM signal at the EN pin of the boost LED driverdevice, as shown in Figure 4.

FIGURE 4: PWM Dimming Method.

The boost LED driver device is turned on and off by thePWM signal and this is the simplest way commonlyused for dimming or adjusting the brightness of theLEDs. With the PWM dimming method, the LEDsoperate in either zero or full current. The average LEDcurrent is proportional to the duty cycle of the PWMdimming signal. This EN pin PWM dimming methodconsumes no current during the off cycle of the PWMdimming signal. However, most of the simple boostLED driver outputs are discharged when the dimmingsignal is low and have to go through the soft start everytime the PWM signal at the EN pin changes fromlow-to-high. Therefore, the period of the PWM dimmingsignal has to be significantly longer than the soft starttime of the boost LED driver; otherwise, the averageLED current will not be proportional to the duty cycle ofthe PWM dimming signal. Therefore, the maximumPWM frequency of this dimming method is limited bythe soft start time. On the other hand, the PWMdimming frequency cannot be too low; otherwise, theflicking of LED brightness will be percepted by thehuman eye, so the dimming frequency should bebetween 100 Hz to 10 kHz. The maximum LED currentat 100% PWM duty at the EN pin is set by Equation 3:

EQUATION 3:

VIN

VIN

EN

SW

OUT

FBGND

PWM

RFB

ILED MAX VREF

RFB-------------=

Where:

VREF = Internal Reference VoltageRFB = Lower Feedback Resistor


AN3263

The average LED current of this EN pin PWM dimmingmethod is approximately changed according to thePWM duty cycle and can be calculated as below:

EQUATION 4:

For EN pin PWM dimming method, the minimum PWMdimming pulse width has to be sufficiently longer thanthe soft start time of the boost LED driver. The frequencyof the PWM dimming signal can be estimated by thefollowing equation:

EQUATION 5:

Design Example 2

Let’s consider a practical application example for usingthe EN pin PWM signal dimming method shown inFigure 4.

The design parameters are given as below:

Since the maximum LED current at 100% PWM duty is40 mA, the value of RFB can be calculated fromEquation 3 as below:

EQUATION 6:

For MIC2289, the soft start time is about 100 µs withoutan external soft start circuit. The frequency of the PWMdimming signal can be calculated as below:

EQUATION 7:

Since the EN pin logic level high is 1.5V, as shown inthe “MIC2289 Data Sheet”, a PWM dimming signal with100 Hz frequency and 2V pulse amplitude should beused. Some bench test results are shown in Figure 5through Figure 9.

FIGURE 5: EN Pin PWM Dimming with 5% Duty.






Max. LED Current: 40 mA

Dimming Method: PWM

Dimming Range: 5% to 100%

ILED

VREF

RFB------------- DPWM

Where:

VREF = Internal Reference VoltageRFB = Lower Feedback ResistorDPWM = Duty Cycle of PWM Dimming Signal

fPWM1

4 tSS---------------- DPWM MIN

RFB

VREF

ILED MAX -------------------------- 95mV

40mA-------------- 2.375===

Where:

RFB = 2.37Ω Selected for Available Value

fPWM1

4 100s------------------------ 0.05 125Hz=


AN3263



Continuous Analog Dimming

Continuous analog dimming control is implemented byapplying an analog DC control voltage through a seriesresistor, R1, to the FB pin of the boost LED driver, asshown in Figure 10.

FIGURE 10: Continuous Analog Dimming Method.

Since the voltage at the FB pin is regulated at theinternal reference voltage in steady state, the voltagedifference between the DC control voltage and the FBpin voltage across the R1 generates a programmablecurrent flowing through another series resistor, R2,between the FB pin and the LED current setting resistorRFB. This current generates an effectively adjustableregulated voltage across the LED current setting resistorRFB. The values of R1 and R2 should be in kΩ range.The LED current of this continuous analog dimmingmethod can be programmable by the following equation:

EQUATION 8:

As shown in Equation 8, the LED current is just equalto the undimmed maximum when the VDC is equal toVREF or the DC input is floating, while the LED currentdecreases linearly as the amplitude of the DC controlvoltage increases above the VREF. On the other hand,the LED current increases linearly as the amplitude ofthe DC control voltage decreases below the VREF.However, if the VREF is a very low voltage and VDCmust be a positive voltage, this section of the VDC isless useful for continuous dimming purposes.

For positive DC control voltage, the VDC is bounded bythe following inequality condition:

EQUATION 9:

Therefore, the maximum DC control voltage whichdims the LED current to zero is given by the equationbelow:

EQUATION 10:

VIN

VIN

EN

SW

OUT

FBGND

RFB

R2

R1

DCEquivalent

ILED

VREF

VDC VREF–

R1----------------------------- R2–

RFB--------------------------------------------------------------=

or

ILED

VREF

RFB-------------

R2R1------- 1+

VDC

RFB----------

R2R1-------–=

Where:

VREF = Internal Reference VoltageRFB = Lower Feedback ResistorVDC = DC Dimming Control VoltageR1 = Series Resistor between VDC and FB PinR2 = Series Resistor between FB pin and RFB

VDC VREFR1R2------- 1+

VDC MAX VREFR1R2------- 1+ =


AN3263

In most applications, VDC is in the range of a few volts,while the VREF is just less than 0.1V. Therefore, thevalue of R1 should be more than ten times larger thanthe value of R2, as indicated by Equation 10. Then, themaximum LED current can be approximated by:

EQUATION 11:

In fact, not every application needs to dim the LEDcurrent to zero. Then, the following more generalequation can be used:

EQUATION 12:

The value of the R2 resistor is in the range of kOhm andthe value of VDC(MAX) is in the range of a few volts.When the value of R2 and VDC(MAX) is selected, thevalue of R1 can be calculated by:

EQUATION 13:

The LED current and brightness can be dynamicallyvaried by applying a DC voltage through the seriesresistor R1 to the FB pin. The DC voltage can be froma DAC signal or RC filtered PWM signal. For an RCfiltered PWM signal applied to the DC terminal, the LEDcurrent can be programmed by the below equation:

EQUATION 14:

For the RC filter used for the filtered PWM signal togenerate the equivalent DC control voltage, the valueof R should be selected with the condition that it will notcause a significant voltage drop to the DC controlvoltage, as the current is drawn into the R1 and R2network, passing through the resistor R of the filter.Therefore, the resistance of the RC filter can be chosenby the following inequality, as shown in Equation 15.

EQUATION 15:

Then, the capacitance of the RC filter can be estimatedby the equation below:

EQUATION 16:

The advantage of this continuous analog dimmingapproach is that the boost LED driver does not need togo through a soft start repeatedly and a high-frequencyPWM signal larger than 10 kHz can be used to controlLED brightness.

Design Example 3

Let’s consider another practical application example forusing the analog DC voltage continuous dimmingmethod, as shown in Figure 10.

The design parameters are given as below:I

The ILED(MAX) is 40 mA at 100% LED current. Thevalue of RFB is calculated from Equation 11 as below:

EQUATION 17:

The ILED(MIN) is 0 mA at 0% LED current. Since the rec-ommended R2 is in kOhm and the recommendedVDC(MAX) is in the range of a few volts, then R2 = 1 kΩ andVDC(MAX) = 2V are chosen, respectively, for convenience.

Then, the value of R1 is calculated from Equation 13 asfollows:

EQUATION 18:

ILED MAX VREF

RFB-------------

R2R1------- 1+ =

ILED MAX VREF

RFB-------------

When R1>>R2:

VDC VREF ILED RFB– R1R2------- VREF+=

R1R2 VDC MAX VREF– VREF ILED MIN RFB–------------------------------------------------------------=

ILED

VREF

RFB-------------

R2R1------- 1+

VPWMA DPWMRFB

----------------------------------------R2R1-------–=

Where:

VPWMA = Amplitude of PWM SignalDPWM = PWM Duty Cycle




Max. LED Current: 40 mA

Dimming Method: Analog DC Voltage

Dimming Range: 0% to 100%

Rf

0.01 VDC MAX R1VDC MAX VFB–

-------------------------------------------------------

Cf5

R f fPWM----------------------------------»

Where:

Rf = Resistance of the RC FilterfPWM = Frequency of the PWM Signal

RFB

VREF

ILED MAX --------------------------

95mV40mA-------------- 2.375==

Where:

RFB = 2.37Ω Selected for Available Value

R11k 2V 95mV–

95mV 0mA 2.37–------------------------------------------------------ 20.05k==

Where:

R1 = 20 kΩ Selected for Commonly Available Value


AN3263

Some bench test results are shown in Figure 11through Figure 16.

FIGURE 11: Analog Continuous Dimming with VDC = 2V, ILED = 0 mA (0%).

FIGURE 12: Analog Continuous Dimming with VDC = 1.9V, ILED = 2 mA (5%).






AN3263

Design Example 4

The same conditions as in the previous “DesignExample 3”, but a filtered PWM signal is used togenerate the equivalent DC control voltage.

The extra design parameters are given as below:

VDC(MAX) ≈ VPWMA at 100% PWM duty.

Using Equation 15, the value of Rf of the filter can bechosen by the below calculated result:

EQUATION 19:

Rf = 100Ω is selected to minimize the voltage drop tothe DC control voltage.

From Equation 16, the value of Cf of the filter can bechosen by the following calculated result:

EQUATION 20:

Cf = 10 µF is selected for obtaining better filteredresulting DC control voltage.

The bench test results are shown in Figure 17 throughFigure 21.

FIGURE 17: Continuous Dimming with Filtered PWM, DPWM = 95%, ILED = 2 mA.

FIGURE 18: Continuous Dimming with Filtered PWM, DPWM = 80%, ILED = 8.4 mA.


FIGURE 20: Continuous Dimming with Filtered PWM, DPWM = 20%, ILED = 33.6 mA.

PWM Signal Frequency (fPWM): 20 kHz

PWM Signal Amplitude (VPWMA): 2V

Rf0.01 2V 20k

2V 95mV–--------------------------------------------

Rf 210

Cf5

100 20kHz----------------------------------------------»

Cf 0.796F»


AN3263


CONCLUSION

In this application note, an external soft start circuit, ENpin PWM LED dimming circuit and continuous analogDC voltage LED dimming circuit for a simple boost LEDdriver, such as MIC2289, are discussed. Also, therelated design equations are introduced and designexamples are demonstrated with test results to assistthe implementation of the circuits.

REFERENCE

1. “MIC2289 Data Sheet – White LED Driver InternalSchottky Diode and OVP”, DS20006207, MicrochipTechnology Inc., 2019.


AN3263

NOTES:


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ISBN: 978-1-5224-5108-2

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