CM9130 Datasheet, PDF (11/12 Pages) California Micro Devices Corp – Three Output Driver for White LEDs

PRELIMINARY
CM9130
Application Information (cont’d)
CM9130 Design Examples
Cell Phone
Some mobile phone LCD displays (both STN and mini-
TFT) use white LEDs for backlighting. Light guides are
used to distribute the light uniformly behind the LCD. A
typical application is shown in Figure 9. The display’s
intensity can be lowered by a PWM signal applied to
the EN pin, as determined by ambient light conditions.
V_BATT
VIN
VOUT
R _SET
EN
LED1
LED2
LED3
ISET
CM9130
GND
Display
MENU
Figure 9. Display Backlight
Camera Flash
The CM9130 can support a camera flash in digital still
cameras as well as in camera equipped smart phones
and PDAs. In this case the flash LEDs are supplied 3 x
50-mA = 150-mA. See Figure 10.
Capacitor Selection
For proper performance, use surface-mount, low ESR
ceramic capacitors for all four positions. X7R or X5R
ceramic dielectric provides good stability over the oper-
ating temperature and voltage range,
The capacitance and ESR of the external bucket
capacitors will directly affect the output impedance and
efficiency of the converter. A ceramic 1μF capacitor is
recommended.
Reflected input ripple depends on the impedance of
the V_IN source, such as the PCB traces and the Li-ion
battery, which have elevated impedance at higher fre-
quencies. The input capacitor located near the con-
verter input reduces this source impedance and ripple.
Any ESR from the capacitor will result in steps and
spikes in the ripple waveform, and possibly produce
EMI. Much of the ripple voltage is due to moving cur-
rent charge in and out of the capacitor and the capaci-
tor’s impedance at the charge pump frequency. If ripple
voltage or current on the battery bus is an application
issue, add a small input inductor between the battery
and the capacitor, or just increase the capacitor.
For a given output current, increasing the output
capacitance reduces output ripple in the 1.5x mode.
Increasing the output capacitor will also increase star-
tup current and time. In most LED applications, high
frequency output ripple is not a concern because it will
not cause intensity variations that are visible to the
human eye.
V_BATT
Flash
R_SET
VIN
EN
ISET
VOUT
LED1
LED2
LED3
CM9130
GND
WLED
Flash
Figure 10. Flash Application
Layout Guide
The charge pump is rapidly charging and discharging
the external capacitors, so external traces to the
capacitors should be made wide and short 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. Use a solid ground plane, and con-
nect the ground side of C_IN, C_OUT and the package
GND as close as practical.
© 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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