CAT4238
DEVICE OPERATION
The CAT4238 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 operating from a Li-Ion battery, the device can
deliver 20mA of load current into a string of up to 10
white LEDs. For higher input voltages, the LED
current can be increased.
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 40V, 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 6μs and keeping it on
for about 1μs.
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 2mA) and with input
voltage above 5.0V, the CAT4238 driving 10 LEDs,
the driver starts pulse skipping. Although the LED
current remains well regulated, some lower frequency
ripple may appear.
Switching Waveform VIN = 5.0V, I
LED
= 1.5mA
Doc. No. MD-5019 Rev. E
4
© 2008 SCILLC. All rights reserved.
Characteristics subject to change without notice
ONSEMI [ ON SEMICONDUCTOR ]
