CM9142 Datasheet, PDF (13/14 Pages) California Micro Devices Corp – Two Group, 4 and 2, WLED Driver, Different Current Settings

PRELIMINARY
CM9142
Application Information (cont’d)
torch light or a preview light in normal operation, and
for full flash when the external switch is turned on. In
this example, the main display intensity is controlled by
two line inputs to ISET1, and the torch light is con-
trolled by S1.
V_BATT
EN2
66K
EN1
33K
15K
R_SET2
S1
VIN
VOUT
ISET1
LED1
LED2
LED3
LED4
CM9142
ISET2
GND
LED5
LED6
Main
Display
MENU
Flash/Torch
Photo Flash
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.
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.
Figure 15. Display, Torch and Full Flash
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.
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