CM9156B Datasheet, PDF (9/13 Pages) California Micro Devices Corp – Charge-Pump White LED Driver

PRELIMINARY
CM9156B
Application Information (cont’d)
When the input voltage is greater then the output volt-
age, then all this sophistication, and the accompanying
power loss, is unnecessary. The smart CM9156B
knows this, and if the input voltage rises above 5V, the
charge pump automatically disables, removing the volt-
age gain stage and the output is then provided directly
through the LDO, regulated at 4.5V. This increases the
efficiency and minimizes chip heating in this operating
condition.
The CM9156B has over-temperature and over-current
protection circuitry to limit device over-stress and fail-
ure during short circuit conditions. An overcurrent con-
dition will limit the output current (approximately
400mA ~ 600mA) and will cause the output voltage to
drop, until automatically resetting after removal of the
excessive current. Over-temperature protection dis-
ables the IC when the junction is about 135-ºC, and
automatically turns-on the IC when the junction tem-
perature drops by approximately 15-ºC.
Efficiency
A conventional charge pump with a fixed gain of 2x will
usually develop more voltage than is needed to drive
paralleled white LEDs from Li-Ion sources. This exces-
sive gain develops a higher internal voltage, reducing
system efficiency and increasing battery drain in porta-
ble devices. A fractional charge pump with a gain of
1.5x is better suited for driving white LEDs in these
applications.
The CM9156B charge pump automatically switches
between 2 conversion gains, 1x and 1.5x, allowing high
efficiency levels over a wide operating input voltage
range. The 1x mode allows the regulated LDO voltage
to pass directly through to the output when sufficient
input voltage is available; the 1.5x charge pump is
enabled only when the battery input is too low to sus-
tain the output load.
At nominal loads, the switching losses and quiescent
current are negligible. If these losses are ignored for
simplicity, the efficiency, , for an ideal 1.5x charge
pump can be expressed as the output power divided by
the input power;
η ≈ ^P----O----^U----^T-
P_IN
For an ideal 1.5x charge pump, I_IN = 1.5 x I_OUT, and the
efficiency may be expressed as;
P_OUT
P_IN
≈
⎜⎜⎝⎛
V_OUT × I_OUT
V_IN ×1.5 × I_OUT
⎟⎟⎠⎞
=
V_OUT
1.5 × V_IN
V_OUT = 4.5V,
∴ η ≈ ^4.5V
1.5 × V_IN
The ideal 2x charge pump can be similarly expressed;
P----_O----U----T-
P_IN
≈
-2---.-0-4---._×--5---VV-----_I--_N--
In 1x mode, when the input voltage is above the output
voltage, the part functions as a linear regulator and the
ideal efficiency is simply Vout/Vin.
The typical conversion efficiency plots for these modes,
with some losses, are shown in Figure 2.
Efficiency
Vout=4.5V
100
85
1X
70
55
40
3.0
CM9156B
dual mode
1.5X
2X
3.5 4.0 4.5 5.0 5.5 6.0
Input Voltage (V)
Figure 2. Ideal efficiency curves
As can be seen, the CM9156B, with 1x and 1.5x
modes, has better efficiency in this application than a
fixed 2x charge pump. At low battery voltages, the
higher efficiency of the CM9156B charge pump’s 1,5x
gain reduces the battery drain. At higher input volt-
ages, above 4.9V typically seen when the system is
running off an AC adapter, the CM9156B, operating the
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