CAT4137 Datasheet, PDF (8/16 Pages) Catalyst Semiconductor – CMOS Boost Converter - White LED Driver

CAT4137
DEVICE OPERATION
The CAT4137 is a fixed frequency (1MHz), low
noise, inductive boost converter providing constant
current to the load. A high voltage internal CMOS
power switch is used to energize the external
inductor. When the power switch is then turned off,
the stored energy inductor is released into the load
via the external 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 external
feedback resistor connected to the feedback pin
(FB). The value of external resistor will accurately set
the LED bias 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 will comfortably
deliver up to 30mA of bias current into a string of up to
5 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 24V, an internal protection circuit will
become active and place the device into a very low
power safe operating mode. In addition, an internal
clamping circuit will limit the peak output voltage to
29V. If this fault condition is repaired, the device will
automatically resume normal operation.
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.
BLOCK DIAGRAM
V_IN
C1
V_IN
SHDN
V_REF
300mV –
+ ^A1
Enable
R_C
C_C
Thermal
Shutdown
& UVLO
1MHz
Oscillator
+
A₂
–
Current
Sense
SW
Over Voltage
Protection
PWM
&
Logic
Driver
N₁
V_OUT
C2
I_LED
R_S
GND
FB
R1
15Ω
Doc. No. MD-5012 Rev. B
8
© Catalyst Semiconductor, Inc.
Characteristics subject to change without notice