CM9156B Datasheet, PDF (10/13 Pages) California Micro Devices Corp – Charge-Pump White LED Driver

PRELIMINARY
CM9156B
Application Information (cont’d)
1x mode, has better efficiency than single mode 1.5x
or 2x charge pumps, lowering the power dissipation for
cooler circuit operation and long life.
LED Brightness Control
CM9156B Design Example
Capacitor Selection
The external bucket capacitors will affect the output
impedance of the converter, so surface-mount, low
ESR ceramic capacitors are recommended. Tantalum
and Aluminum capacitors should not be used because
their ESR is too high. The ceramic dielectric must be
stable over the operating temperature and voltage
range, X7R or X5R dielectrics are recommended. In
noise sensitive applications, output ripple can be fur-
ther reduced by increasing the capacitance of the out-
put capacitor. Reflected input ripple current depends
on the impedance of the VIN source, which includes
the PCB traces. Increasing the input capacitor will
reduce this ripple. The input capacitor also affects the
output voltage ripple. All the capacitors should be
located close to the device for best performance.
Frequency Selection
The optimal switching frequency depends on the allow-
able system current draw, the load current, ripple and
EMI requirements. The CM9156B’s operating fre-
quency choices are 262kHz or 650kHz. These two fre-
quencies are selected by programming the CLK input.
Refer to Table 1. The supply current for a charge pump
is proportional to its switching frequency. A lower
switching frequency allows reduced quiescent current
for more efficient operation, but reduces the output cur-
rent capability and in some cases, causes higher rip-
ple. Higher frequencies are used when larger load
currents are demanded.
The frequency is typically selected to achieve maxi-
mum efficiency while avoiding sensitive frequencies
with the switching fundamental and its harmonics. The
switching frequency can be set outside the critical fre-
quency spectrums of cellular communications band-
widths. Once set, the switching frequency and its
harmonics remain fixed, making filtering easy.
VOUT C 2P
V in
C1P C1N
VIN
GND
C 2N
C LK ENA
CM9156B
PWM input
Figure 3. PWM brightness control, lowered
quiescent current
Changes in ambient light often require the backlight
display intensity to be adjusted, usually to conserve
battery life. There are simple solutions to lowering the
LED brightness when using the CM9156B.
A PWM signal applied to the ENA pin can be used to
control the brightness, which is more efficient than
other solutions that dissipate unwanted LED current in
the series resistors. It also maintains the white LED
color fidelity by avoiding color temperature variations
that come with bias current changes. The LED intensity
is determined by the PWM duty cycle, which can vary
from 0% to 100%.
In the configuration shown in Figure 3, the brightness
is controlled by the PWM signal applied to the LEDs.
Decreased Duty Cycle will lower the LED brightness,
See Figure 4 and Figure 5. The same signal is also
applied to the CM9156B, reducing the charge pump
switching frequency via the CLK control. When the
PWM signal is high, CLK goes high, the operating fre-
quency is 650kHz (refer to Table 1), and the LED cur-
rent 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 frequency of the
charge pump becomes 262kHz (CLK = 0). Operating
the charge pump at the lower frequency lowers the qui-
escent current when the charge pump is operational
(the input voltage below 5V).
© 2006 California Micro Devices Corp. All rights reserved.
10 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: [phone] l Fax: [phone] l www.cmd.com
04/26/06