A High-Efficiency Step-down LED Driver for Fire Alarms

Chris Glaser, Member Group Technical Staff, Applications Engineer

Texas Instruments

Mouser Building Automation Webinar

November 2016

1

LED Driver Theory

3

How to Drive LEDs?

What does this do?

TPS621xx

Design Challenges

• Dimming (analog or PWM) usually required

• Sense resistor is a loss term should be minimize-able

• Wide Vin range (low and high), ~3A LED current, small (but thermally good)

package required very few ICs available

5

Step-Down LED Driver with Dimming (PMP9762)

• 4.5V to 17V input converter that efficiently drives one high power LED at

currents up to 3A

• Supports analog or PWM dimming

• For use with the TPS62130 (3A), TPS62140 (2A), TPS62150 (1A), and

TPS62135 (4A) devices all wide Vin step-down converters

• Full design equations in SLVA451 and SLVA688

Basic Schematic for 1A LED Current

This resistor, RSS, lowers the voltage at the FB pin to minimize

the losses in the sense resistor, RCS

Analog or PWM dimming

How Does it Work?

Iss

This Voltage

Internally Clamped

to 1.25V

Clamped Voltage

Gained Down to 0.8V

Full Scale at FB Pin

Inside IC Current Source

Develops Voltage

Across Rss

Dimming Techniques

9

Analog Dimming

• A voltage applied to the SS/TR pin controls the FB pin voltage and thus the LED

current

• Provides excellent dimming linearity

• Provides very high efficiency

• Due to offset at very low SS/TR pin voltages, cannot turn off LEDs completely

through the SS/TR pin

– Must use the EN pin if complete disable function is desired

• Can be used to trim the light output of an individual fire alarm regulations!

0

0.025

0.05

0.075

0.1

0.125

0.15

0.175

0.2

0.225

0.25

0.275

0

100

200

300

400

500

600

700

800

900

1000

1100

0 50 100 150 200 250 300 350 400

Vfb

(V

)

LE

D C

urr

en

t (m

A)

VSS (mV)

Vin=17V

Vin=4V

Vfb

Analog Dimming Linearity

At 0 mV on SS/TR, 5% of full LED current still flows in the LED

PWM Dimming

• An alternative to analog dimming, PWM dimming eliminates the color shift that

occurs at different LED currents

• PWM dimming operates the LED at full current always

• Duty cycle of the applied PWM signal dictates perceived average light level

– Frequency of dimming should remain high enough to not be visible, yet low enough to

keep dimming linearity

• 100 Hz recommended

• Fire alarms generally use a ~1 Hz PWM ‘dimming’ to create a strobe light

0

100

200

300

400

500

600

700

800

900

1000

1100

0 10 20 30 40 50 60 70 80 90 100LE

D C

urr

en

t (m

A)

PWM Duty Cycle (%)

Vin=17V

Vin=4V

PWM Dimming Linearity on EN Pin

At 0% duty cycle on EN pin, 0 mA flows in the LED LED is off

What Happens if I PWM Dim Higher than 100 Hz?

PWM Dimming Linearity vs. Frequency

0

100

200

300

400

500

600

700

800

900

1000

1100

100 1000 10000

LE

D C

urr

en

t (m

A)

PWM Frequency (Hz)

D = 10%

D = 50%

D = 90%

D=10%,ideal

Ideal Current

Real Current

Deviation from Ideal Current vs. Frequency

-100

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

100 1000 10000

De

via

tio

n f

rom

Id

ea

l C

urr

en

t (%

)

PWM Frequency (Hz)

D = 10%

D = 50%

D = 90%

Dimming Comparison

• Both analog and PWM dimming provide excellent linearity

• Analog dimming requires the use of the EN pin to completely turn off the LED

• Efficiency is better with analog dimming due to lower forward voltage of LED

PMP9762 Reference Design Performance

18

What is needed in a fire alarm?

• Regulations! Public Safety!

– Specific and controlled light output – not too little and not too much varies by

country and application must be adjustable

– Synchronization to other fire alarms and/or a system ‘clock’

– Reliability

– Robustness

• ~12V input voltage, but this may not be well-regulated wide input voltage

range required

• Small size to fit inside the fire alarm with the other electronics (audio, MCU, etc.)

• Usually, one bright LED is used instead of multiple smaller ones reliability

– Step-down LED driver needed not common in the market!

19

PMP9762 Performance (from test report SLVA688)

20

Efficiency

21

Why is it measured ‘pulsed’?

Thermal Performance

22

~40 °C LED

temp rise at

1A

~20 °C IC

temp rise

at 1A

LED dissipates much more power and thermally limits the system at DC

PMP9762 BOM

23

Just 8 total components (including the LED)!

Integrated FETs and no external compensation

Simple circuit!!

PMP9762 LED Driver Summary

• 3A LED currents supported with the TPS62130

– 4A with the new TPS62135

• Analog and PWM dimming supported strobe effect, LED current trimming

• Nearly 90% efficient LED driver with very good thermal package low self

temp rise

• Simple design with just 8 components small total solution

• Wide input voltage range 4.5V to 17V supports quasi-regulated 12Vin

systems

• Proven hardware reference design: http://www.ti.com/tool/PMP9762