CAT4240

Doc. No. MD-5026 Rev. C

6

© Catalyst Semiconductor, Inc.

Characteristics subject to change without notice

PIN DESCRIPTION

VIN is the supply input for the internal logic. The

device is compatible with supply voltages down to

2.8V and up to 5.5V. It is recommended that a small

bypass ceramic capacitor (4.7μF) be placed between

the VIN and GND pins near the device. If the supply

voltage drops below 1.9V, the device stops switching.

SHDN

¯¯¯¯¯ is the shutdown logic input. When the pin is

tied to a voltage lower than 0.4V, the device is in

shutdown mode, drawing nearly zero current. When

the pin is connected to a voltage higher than 1.5V, the

device is enabled.

GND is the ground reference pin. This pin should be

connected directly to the ground plane on the PCB.

SW pin is connected to the drain of the internal CMOS

power switch of the boost converter. The inductor and

the Schottky diode anode should be connected to the

SW pin. Traces going to the SW pin should be as

short as possible with minimum loop area. An over-

voltage detection circuit is connected to the SW pin.

When the voltage reaches 40V, the device enters a

low power operating mode preventing the SW voltage

from exceeding the maximum rating.

FB feedback pin is regulated at 0.3V. A resistor

connected between the FB pin and ground sets the

LED current according to the formula:

The lower LED cathode is connected to the FB pin.

Pin #

Name

Function

1

SW

Switch pin. This is the drain of the internal power switch.

2

GND

Ground pin. Connect the pin to the ground plane.

3

FB

Feedback pin. Connect to the last LED cathode.

4

SHDN

¯¯¯¯¯

Shutdown pin (Logic Low). Set high to enable the driver.

5

VIN

Power Supply input.

SIMPLIFIED BLOCK DIAGRAM

Current

Sense

Ref

300mV

LED

Current

C1

SHDN

Thermal

Shutdown

& UVLO

1MHz

Oscillator

Over Voltage

Protection

PWM

&

Logic

Driver

C2

GND

SW

FB

R1

+

–