CM9156B Datasheet, PDF (11/13 Pages) California Micro Devices Corp – Charge-Pump White LED Driver

PRELIMINARY
CM9156B
Application Information (cont’d)
C2N, 5V/div
PWM, 20 kHz,
60% D.C., 5V/div
Vout ripple,
200 mV/div
Iin, 200 mA/div
Vin=3.8V 20 uSec/div
Figure 4. High brightness waveforms
Camera Flash Application
Many smart phones and PDAs include a digital cam-
era. These cameras typically utilize a WLED flash to
illuminate the picture subject in low light conditions.
The CM9156B is easily adapted to such an application.
Figure 6 is a typical application using the CM9156B as
a WLED flash driver, which is ideal for this application
because it is capable of driving up to 180mA from a Li-
ion battery. The One-shot is used to create a single
pulse of a set duration to the ENA pin of the CM9156B.
The Flash LED modules shown here contain three
matched WLEDs with a common anode and separated
cathodes. The series resistor is chosen based on the
forward drop of the module LEDs (typically 3.3V to
3.8V) and the number of parallel LEDs being driven.
The recommended PWM frequency is between 100 Hz
and 20kHz. If a frequency of less then 100 Hz is used,
flicker might be seen in the LEDs. The frequency
should also be greater than the refresh rate of the TFT
display. Higher frequencies will cause a loss of bright-
ness control linearity. In addition, higher frequency can
cause chromaticity shifts because the fixed rise and fall
times of the PWM signal will shift the forward current.
C2N, 5V/div
VOUT C 2P
C1P C1N
V in
VIN
GND
C 2N
R_{CAT HODE}
CLK
E NA
CM9156B
R
CAT HODE
Flash
One- shot
Pulse
t
Figure 6. Camera flash application
PWM, 20 kHz,
10% D.C., 5V/div
Vout ripple,
200 mV/div
Iin, 200 mA/div
Vin=3.8V 20 uSec/div
Figure 5. Low brightness waveforms
Layout Guide
The charge pump is rapidly charging and discharging
its external capacitors, so external traces to the capac-
itors should be made as wide and short as allowable to
minimize inductance and high frequency ringing. The
four capacitors should be located as close as practical
to the charge-pump, particularly C1 and C2, which
have the highest dv/dt. Connect ground and power
traces to the capacitors through short, low impedance
paths. Use a solid ground plane, ideally on the back-
side of the PCB, which should carry only ground poten-
tial. Connect the ground-side of Cin, Cout and the chip
GND as close as practical. For best thermal perfor-
mance, the exposed backside lead frame should be
soldered to the PCB.
© 2006 California Micro Devices Corp. All rights reserved.
04/26/06 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: [phone] l Fax: [phone] l www.cmd.com
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