CAT4237TD-T3 Datasheet, PDF (4/14 Pages) ON Semiconductor – High Voltage CMOS Boost White LED Driver

CAT4237
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 60µs 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.
Figure 1. Switching Waveform V_IN = 4.2V,
I_LED = 4mA
Doc. No. MD-5013, Rev. C
4
2008 SCILLC. All rights reserved.
Characteristics subject to change without notice