CAT32_08 Datasheet, PDF (9/14 Pages) Catalyst Semiconductor – CMOS White LED Driver

CAT32
LED DIMMING WITH A LOGIC SIGNAL
For applications that need to adjust the LED
brightness in discrete steps, a logic signal can be
used. RMIN sets the minimum LED current value
(when the NMOS is OFF):
0.1V
R_MIN = 255 × _ILED(MIN)
R_INCR determines how much LED current increases
when the external NMOS switch is turned ON.
0.1V
R_INCR = 255 × _{ILED(Increase)}
LED DIMMING WITH A DC VOLTAGE
PCB Layout Guidelines
The CAT32 is a high-frequency switching regulator
and therefore proper PCB board layout and
component placement can minimize noise and
radiation and increase efficiency. To maximize
efficiency, the CAT32 design has fast switch rise and
fall times. To prevent radiation and high frequency
resonance problems minimize the length and area of
all traces connected to the SW pin and use a ground
plane under the switching regulator.
The switch, schottky output diode and output
capacitor signal path should be kept as short as
possible. The ground connection for the R_SET resistor
should be tied directly to the GND pin and not be
shared with other components.
R_ADJ = 225 ×
V_MAX – 0.1V
I_LED(MAX) – I_LEAD(MIN)
CAT32
SHDN
5
PWM
CAT32
RSET
4
R_SET
R_PWM
PWM
CAT32
RSET
4
R_SET
10kΩ
R_PWM
PWM
0.1µF
CAT32
RSET
4
R_SET
R_ADJ
CAT32
V_DC
RSET
4
R_MIN
R_INCR
Logic
Signal
Figure 3. LED Dimming Circuits
© Catalyst Semiconductor, Inc.
9
Characteristics subject to change without notice
Doc. No. MD-5001 Rev. AD