CAT4237TS datasheet(4/13 Pages) CATALYST

Part #	CAT4237TS

Description	High Voltage CMOS Boost White LED Driver

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Manufacturer	CATALYST [Catalyst Semiconductor]

Direct Link	http://www.catalyst-semiconductor.com

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CAT4237

4 of 13

Doc. No. 5013, Rev. A

Characteristics subject to change without notice

DEVICE OPERATION

The CAT4237 is a fixed frequency (1MHz),

low noise, inductive boost converter that

provides a constant current with excellent

line and load regulation. The device uses a

high-voltage CMOS power switch between

the SW pin and ground to energize the

inductor. When the switch is turned off, the

stored energy in the inductor is released into

the load via the Schottky diode.

The on/off duty cycle of the power switch is

internally adjusted and controlled to maintain

a constant regulated voltage of 0.3V across

the feedback resistor connected to the

feedback pin (FB). The value of the resistor

sets the LED current accordingly (0.3V/R1).

During the initial power-up stage, the duty

cycle of the internal power switch is limited

to prevent excessive in-rush currents and

thereby

provide

a

“soft-start”

mode

of

operation.

While in normal operation, the device can

deliver up to 40mA of load current into a

string of up to 8 white LEDs.

In the event of a “Open LED” fault condition,

where the feedback control loop becomes

open, the output voltage will continue to

increase. Once this voltage exceeds 35V, an

internal protection circuit will become active

and place the device into a very low power

safe operating mode where only a small

amount of power is transferred to the output.

This is achieved by pulsing the switch once

every 60

µs and keep it on for about 1µs

only.

Thermal overload protection circuitry has

been included to prevent the device from

operating at unsafe junction temperatures

above 150ºC.

In the event of a thermal

overload

condition

the

device

will

automatically shutdown and wait till the

junction temperatures cools to 130ºC before

normal operation is resumed.

Light load operation

Under light load condition (under 4mA) and

with input voltage above 4.2V, the CAT4237

driving 6 LEDs, the driver starts pulse

skipping. Although the LED current remains

well regulated, some lower frequency ripple

may appear.

Switching Waveform VIN = 4.2V, ILED = 4mA

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CAT4237TS Datasheet(PDF) 4 Page - Catalyst Semiconductor

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