CAT4238
PIN DESCRIPTION
VIN
is the supply input for the internal logic. The
device is compatible with supply voltages down to
2.8V and up to 5.5V. It is recommended that a small
bypass ceramic capacitor (4.7μF) be placed between
the VIN and GND pins near the device. If the supply
voltage drops below 1.9V, the device stops switching.
¯¯¯¯¯
SHDN
is the shutdown logic input. When the pin is
tied to a voltage lower than 0.4V, the device is in
shutdown mode, drawing nearly zero current. When
the pin is connected to a voltage higher than 1.5V, the
device is enabled.
GND
is the ground reference pin. This pin should be
connected directly to the ground plane on the PCB.
SW
pin is connected to the drain of the internal CMOS
power switch of the boost converter. The inductor and
the Schottky diode anode should be connected to the
SW pin. Traces going to the SW pin should be as
short as possible with minimum loop area. An over-
voltage detection circuit is connected to the SW pin.
When the voltage reaches 40V, the device enters a
low power operating mode preventing the SW voltage
from exceeding the maximum rating.
FB
feedback pin is regulated at 0.3V. A resistor
connected between the FB pin and ground sets the
LED current according to the formula:
ILED = 0.3V/R1
The lower LED cathode is connected to the FB pin.
Pin #
1
2
3
4
5
Name
SW
GND
FB
¯¯¯¯¯
SHDN
VIN
Function
Switch pin. This is the drain of the internal power switch.
Ground pin. Connect the pin to the ground plane.
Feedback pin. Connect to the last LED cathode.
Shutdown pin (Logic Low). Set high to enable the driver.
Power Supply input.
BLOCK DIAGRAM
47µH
VIN
C1
4.7µF
1MHz
Oscillator
VIN
–
Vref
300mV
A1
Enable
Thermal
Shutdown
& UVLO
+
RC
C
C
R
S
+
GND
Over Voltage
Protection
Driver
N1
LED
Current
SW
C2
0.22µF
+
–
A2
PWM &
Logic
SHDN
Current
Sense
© 2008 SCILLC. All rights reserved.
Characteristics subject to change without notice
3
–
FB
R1
15Ω
Doc. No. MD-5019 Rev. E
ONSEMI [ ON SEMICONDUCTOR ]
