CAT37

6

Doc. No. 5005, Rev. R

OPERATION

The CAT37 device is a high efficiency, constant

frequency, current regulating boost driver for white LEDs.

The device includes a switch and an internally

compensated loop for the regulation of the current in the

LEDs.

Operation can be best understood by examining the

block diagram. The FB pin is regulated at 95 mV and the

current through the external resistor will set the regulated

current in the LEDs at:

While maintaining LED current regulation, the CAT37

boost converter automatically adjusts the FB pin voltage

to be as low as possible. A low FB pin voltage ensures

high efficiency.

Current through the internal power switch is continuously

monitored cycle-by-cycle. If the current limit is exceeded,

the switch is immediately turned off, protecting the

device, for the remainder of the cycle.

PWM dimming operation can be achieved by switching

the

SHDN pin or by pulling the FB pin higher than 95mV.

BLOCK DIAGRAM

Figure 2. CAT37 Block Diagram

VIN

1.2 MHz

Clock

PWM

& Logic

D1

L1

VIN

C1

2

FB

Current

Sense

Amp

SW

1

5

CAT37

3

Error

Amp

C2

4

SHDN

GND

0.095V

REF

ILED

R1

+

-

Over Voltage

Protection

(22V)

Note: SOT23 Pin Numbers