LINEAR Wide Input Range 1A LED Driver Powers High Brightness LEDs with Automotive and 12VAC Supplies – Design Note 388 by John Tilly and Awo Ashiabor

Wide Input Range 1A LED Driver Powers High Brightness LEDs with Automotive and 12VAC Supplies – Design Note 388 John Tilly and Awo Ashiabor Introduction Driving LEDs from 12VAC Input Today’s ultrabright LEDs far exceed the performance The LT3474 directly regulates LED current, maintain- of incandescent bulbs in both effi ciency and lifetime. ing constant LED current over changing VIN. The wide Taking full advantage of these features requires a cor- input range of the LT3474 allows direct connection to respondingly effi cient and reliable LED driver, such as the a rectifi ed 12VAC input. Using a small input capacitor, LT®3474. The LT3474 is a step-down 1A LED driver that as shown in Figure 2, minimizes size. In this case, the supports a variety of power sources, has a wide 4V to LT3474 delivers nearly 1A of LED current as shown in 36V input voltage range and is programmable to deliver Figure 3. Adding more capacitance to the input, as shown LED current from 35m A to 1A at up to 88% effi ciency. It in Figure 4, holds the input voltage above the LED volt- requires minimal external circuitry and is available in a age. In this case, the LT3474 can deliver a constant LED space saving 16-lead TSSOP package. current even with signifi cant 120Hz ripple on the input

as shown in Figure 5.

Automotive LED Driver

, LTC and LT are registered trademarks of Linear Technology Corporation.

Figure 1 shows the confi guration of the LT3474 operating

All other trademarks are the property of their respective owners.

from a 12V automotive battery input. As shown, the circuit can tolerate voltage swings from 4V to 36V, common in an automotive environment. With an integrated NPN

12VAC

switch, boost diode and sense resistor, the LT3474 cuts 0.22µF

VIN BOOST

the external component count to a minimum. The high 6.8µH

SHDN SW

side sense allows a grounded cathode connection, easing

LT3474

wiring constraints. Both PWM and analog dimming are

RT BIAS

available with minor circuit modifi cation; see the LT3474

REF OUT

data sheet for details.

VC LED 1µF

VIN 0.22µF

4V TO 36V

VIN BOOST DN388 F02

SHDN SW

LT3474 Figure 2. Using a Diode Bridge Allows the LT3474 to Drive

RT BIAS an LED from a 12VAC Input REF OUT VADJ

VC LED 2.2µF

0.1µF 10V/DIV

DN388 F01

500m A/DIV

DN388 F03

2ms/DIV

Figure 1. 4V-36V Input Voltage 1A LED Driver Requires

Few Components Figure 3. Using a 10µF Input Capacitance, the LT3474 Delivers Nearly 1A of LED Current with Smallest Board Size