CAT4237
http://onsemi.com
4
Block Diagram
Enable
Current
Sense
300 mV
R1
15 W
LED
Current
4.7 mF
C1
Thermal
Shutdown
& UVLO
1 MHz
Oscillator
Over Voltage
Protection
PWM &
Logic
Driver
0.22 mF
C2
GND
SW
FB
33 mH
+
–
+
–
+
–
A1
A2
V
IN
V
IN
V
REF
SHDN
R
C
C
C
R
S
N
1
Figure 2. Block Diagram
Device Operation
The CAT4237 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
current accordingly (0.3 V/R
1
).
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 ms and
keep it on for about 1 ms 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 CAT4237 driving 6 LEDs, the
driver starts pulse skipping. Although the LED current
remains well regulated, some lower frequency ripple may
appear.
Figure 3. Switching Waveform V
IN
= 4.2 V,
I
LED
= 4 mA
