PRELIMINARY
CM9156A
Application Information (cont’d)
Decreased Duty Cycle will lower the LED brightness,
See
and
The same signal is also
applied to the CM9156A, reducing the charge pump
switching frequency via the CLK2 control. When the
PWM signal is high, CLK1 is low, CLK2 goes high, the
operating frequency is 262kHz (refer to
and
the LED current path is complete through the switch.
When the PWM signal is low, the LED current is
stopped as the switch turns off, and the switching fre-
quency of the charge pump becomes 32kHz (CLK1,
CLK2 = 0). Operating the charge pump at the lower fre-
quency lowers the quiescent current when the charge
pump is operational (the input voltage below 5V).
The recommended PWM frequency is between 100Hz
and 20kHz. If a frequency of less then 100Hz 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
PWM, 20 kHz,
10% D.C., 5V/div
Vout ripple,
200 mV/div
Iin, 200 mA/div
Vin=3.8V
20 uSec/div
Figure 6. Low 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 CM9156A is easily adapted to such an application.
is a typical application using the CM9156A as
a WLED flash driver, which is ideal for this application
because it is capable of driving up to 120mA 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 CM9156A.
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.
Vin=3.8V
20 uSec/div
C2N, 5V/div
PWM, 20 kHz,
60% D.C., 5V/div
Vout ripple,
200 mV/div
Iin, 200 mA/div
Figure 5. High brightness waveforms
©
2006 California Micro Devices Corp. All rights reserved.
04/26/06
490 N. McCarthy Blvd., Milpitas, CA 95035-5112
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Tel: 408.263.3214
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Fax: 408.263.7846
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www.cmd.com
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CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
