PWM

Control

Error Amp

Slope

Comp.

1 MHz

Osc.

1:

1:

+

-

Reference

Soft-start

200 mV

500 mV

2.7V to 5.5V

IN

SW

3W

OUT

Body Diode

Control

OVP

Comp

REF

Thermal

Shutdown

ILED

Current Sense/

Current Limit

SET

AGND

PGND

DIM

EN

LM3508

SNVS494C – APRIL 2007 – REVISED MAY 2013

www.ti.com

OPERATION

Figure 27. LM3508 Block Diagram

The LM3508 utilizes a synchronous step-up current mode PWM controller and a regulated current sink to provide

a highly efficient and accurate LED current for white LED bias. The internal synchronous rectifier increases

efficiency and eliminates the need for an external diode. Additionally, internal compensation eliminates the need

for external compensation components resulting in a compact overall solution.

Figure 27 shows the detailed block diagram of the LM3508. The output of the boost converter (OUT) provides

power to the series string of white LED’s connected between OUT and ILED. The boost converter regulates the

voltage at ILED to 500mV. This voltage is then used to power the internal current source whose output is at SET.

The first stage of the LM3508 consists of the synchronous boost converter. Operation is as follows: At the start of

each switching cycle the oscillator sets the PWM controller. The controller turns the low side (NMOS) switch on

and the synchronous rectifier (PMOS) switch off. During this time current ramps up in the inductor while the

output capacitor supplies the current to the LED’s. The error signal at the output of the error amplifier is

compared against the sensed inductor current. When the sensed inductor current equals the error signal, or

when the maximum duty cycle is reached, the NMOS switch turns off and the PMOS switch turns on. When the

PMOS turns on, the inductor current ramps down, restoring energy to the output capacitor and supplying current

to the LED’s. At the end of the clock period the PWM controller is again set and the process repeats itself. This

action regulates ILED to 500mV.

The second stage of the LM3508 consists of an internal current source powered by the ILED voltage and

providing a regulated current at SET. The regulated LED current is set by connecting an external resistor from

TYPICAL PERFORMANCE CHARACTERISTICS of SET voltage vs DIM frequency). The PWM signal at DIM

modulates the internal 200mV reference and applies it to an internal RC filter resulting in an adjustable SET

voltage and thus an adjustable LED current.

10

Submit Documentation Feedback

Copyright © 2007–2013, Texas Instruments Incorporated

Product Folder Links: LM3508