CM9142 Datasheet, PDF (8/14 Pages) California Micro Devices Corp – Two Group, 4 and 2, WLED Driver, Different Current Settings

PRELIMINARY
CM9142
Application Information (cont’d)
the system efficiency and increasing battery drain in
portable devices. A fractional charge pump with a gain
of 1.5x is better suited for driving white LEDs in these
applications.
The CM9142 charge pump automatically switches
between the two conversion gains, 1x and 1.5x, allow-
ing high efficiency levels over a wide operating input
voltage range. The 1x mode allows the voltage to pass
directly through to the output when sufficient input volt-
age is available. As the battery discharges to the point
where any one current source no longer has sufficient
voltage headroom to maintain a constant current regu-
lation, the 1.5x charge pump is enabled.
At nominal loads, the switching losses and quiescent
current are negligible. If these losses are ignored for
simplicity, the efficiency, η, for an ideal 1.5x charge
pump can be expressed as the output power divided by
the input power:
η = -P---^L---^E----^D-
P_IN
For an ideal 1.5x charge pump, I_IN 1.5 x I_OUT, and the
efficiency may be expressed as;
V_OUT = (V_LED + V_{CURRENT _ SINK} )
P_LED
P_IN
≈
⎜⎜⎝⎛
(V_OUT ) × I_OUT
V_IN ×1.5 × I_OUT
⎟⎟⎠⎞
=
V_OUT
1.5 × V_IN
For (V_LED + V_{CURRENT _ SINK} ) = 3.9V,
η ≈ ^3.9V
1.5 × V_IN
Many charge pumps are fixed 2x designs. The ideal 2x
charge pump efficiency can be similarly expressed;
P----_O----U----T-
P_IN
≈
------3---.--9---V---------
2.0 × V_IN
In 1x mode, when the input voltage is above the output
voltage, the ideal efficiency is simply V_OUT/V_IN.
The typical conversion efficiency plots for these modes,
with some losses, are shown in Figure 2.
VLED=3.5V
90
1X
75
1X-1.5X
dual mode
60
45
1.5X
2X
30
3.0 3.5 4.0 4.5 5.0 5.5 6.0
Input Voltage (V)
Figure 2. Ideal charge pump efficiency
As can be seen, the CM9142, with 1x and 1.5x modes,
has better efficiency in this application than a fixed 2x
charge pump. At low battery voltages, the higher effi-
ciency of the CM9142 charge pump’s 1,5x gain
reduces the battery drain. At higher input voltages, typ-
ically seen when the system is running off an AC
adapter, the CM9142, operating the 1x mode, has bet-
ter efficiency than single mode 1.5x or 2x charge
pumps, lowering the power dissipation for cooler circuit
operation and long life.
While the charge pump efficiency is easily determined,
the system efficiency is more difficult due to the current
source outputs, which complicate measuring the output
power. The forward voltage of the white LEDs will vary,
and the constant current sources will adjust to maintain
the current. When comparing systems, it is best to
compare the input current for a specified LED drive
current.
The 1x mode has better efficiency than the 1.5x mode.
Selecting LEDs with low forward voltage (V_LED)
increases the time spent in the 1x mode as the battery
discharges, extending the operating time.
Failed LED Detection
If a LED is shorted, the CM9142 will continue to oper-
ate and drive the remaining LEDs at the programmed
current. If a LED opens, the other LEDs will still be reg-
ulated at the programmed current.
© 2006 California Micro Devices Corp. All rights reserved.
8 490 N. McCarthy Blvd., Milpitas, CA 95035-5112 l Tel: [phone] l Fax: [phone] l www.cmd.com
04/26/06