W-6237

4

Block Diagram

Enable

Current

Sense

300 mV

R1

15

LED

Current

4.7 F

C1

Thermal

Shutdown

& UVLO

1 MHz

Oscillator

Over Voltage

Protection

PWM &

Logic

Driver

0.22 F

C2

GND

SW

FB

33 H

+

–

+

–

A1

A2

SHDN

Figure 2. Block Diagram

Device Operation

The W-6237 is a fixed frequency (1 MHz), 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 0.3 V across the feedback resistor connected to the

feedback pin (FB). The value of the resistor sets the LED

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

40 mA of load current into a string of up to 8 white LEDs.

In the event of an “Open LED” fault condition, where the

feedback control loop becomes open, the output voltage will

continue to increase. Once this voltage exceeds 35 V, 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 s and

keep it on for about 1 s only.

Thermal overload protection circuitry has been included

to prevent the device from operating at unsafe junction

temperatures above 150 C. In the event of a thermal

overload condition the device will automatically shutdown

and wait till the junction temperatures cools to 130 C before

normal operation is resumed.

Light Load Operation

Under light load condition (under 4 mA) and with input

voltage above 4.2 V, the W-6237 driving 6 LEDs, the

driver starts pulse skipping. Although the LED current

remains well regulated, some lower frequency ripple may

appear.