4

LT1932

BLOCK DIAGRA

–

+

1

3

Σ

×5

1.2MHz

OSCILLATOR

DRIVER

S

Q

R

–

+

A2

–

+

A1

0.04

Ω

+

+

Q1

Q2

LED

SW

D1

6

L1

5

SHDN

C1

C2

DRIVER

LED CURRENT

REFERENCE

4

1932 F01

2

GND

Figure 1. LT1932 Block Diagram

OPERATIO

The LT1932 uses a constant frequency, current mode

Operation can be best understood by referring to the

block diagram in Figure 1. At the start of each oscillator

cycle, the SR latch is set, turning on power switch Q1. The

signal at the noninverting input of the PWM comparator

A2 is proportional to the switch current, summed to-

gether with a portion of the oscillator ramp. When this

signal reaches the level set by the output of error amplifier

A1, comparator A2 resets the latch and turns off the

power switch. In this manner, A1 sets the correct peak

current level to keep the LED current in regulation. If A1’s

output increases, more current is delivered to the output;

if it decreases, less current is delivered. A1 senses the

LED current in switch Q2 and compares it to the current

than 100mV will pull down the output of A1, turning off

power switch Q1 and LED switch Q2.

PI FU CTIO S

SW (Pin 1): Switch Pin. This is the collector of the internal

NPN power switch. Minimize the metal trace area con-

nected to this pin to minimize EMI.

GND (Pin 2): Ground Pin. Tie this pin directly to local

ground plane.

LED (Pin 3): LED Pin. This is the collector of the internal

NPN LED switch. Connect the cathode of the bottom LED

to this pin.

programs the LED current (that flows into the LED pin).

This pin is also used to provide LED dimming.

SHDN (Pin 5): Shutdown Pin. Tie this pin higher than

0.85V to turn on the LT1932; tie below 0.25V to turn it off.

capacitor to ground as close to the device as possible.