PRELIMINARY
CM9156A
Application Information
The CM9156A is a switched capacitor charge pump
voltage converter ideally suited for driving white LEDs
to backlight or sidelight LCD color displays for portable
devices such as cellular phones, PDAs and any appli-
cation where small space and efficiency are critical.
The CM9156A charge pump is the perfect driver for
such portable applications, providing efficiency, com-
pact overall size, low system cost and minimal EMI.
The CM9156A contains a linear low dropout (LDO)
regulator followed by a 1.5x fractional charge pump
that converts the nominal lithium-ion (Li-Ion) or lithium
polymer battery voltage levels (3.6V) by a gain of 1.5
times and regulates the converted voltage to 4.5V,
±
-
5%, enough to drive the forward voltage drop of white
LEDs. The CM9156A requires only two external
switched, or bucket, capacitors plus an input and an
output capacitor, providing for a compact, low profile
design. In many applications, all these can conve-
niently be the same value of 1.0μF, commonly available
in a compact 0805 surface mount package.
The CM9156A is intended for white LED applications,
but it can drive most all types of LEDs with a forward
voltage drop of less than 4V.
The LED current is determined by its series resistor,
R
LED
, and is approximately;
I
LED
=
4.5 V
−
V
FWD _ LED
R
LED
The input voltage, VIN, passes through an LDO pre-
regulator that compares the output voltage to a
precision bandgap reference. After the LDO, the
charge pump boosts the LDO voltage by 1.5 times. A
feedback circuit to the LDO monitors the output
voltage, and when the output voltage reaches 4.5V, the
LDO output will operate at about 3V, regulating the
device output at 1.5 x 3V = 4.5V.
The charge pump uses two phases from the oscillator
to drive internal switches that are connected to the
bucket capacitors, C1 and C2, as shown in
In
the first switch position, the bucket capacitors are con-
nected in series and each are charged from the LDO to
a voltage of V
LDO
/2. The next phase changes the
switch positions so that C1 and C2 are put in parallel,
and places them on top of V
LDO
. The resulting voltage
across C
OUT
is then; V
LDO
+1/2V
LDO
= 1.5 x V
LDO
.
When the input voltage rises above 5V, the charge
pump automatically disables, removing the voltage
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 CM9156A has over-temperature and over-current
protection circuitry to limit device stress and failure dur-
ing short circuit conditions. An overcurrent condition
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 disables the IC
when the junction is about 135
°
C, and automatically
turns on the IC when the junction temperature drops by
approximately 15
°
C.
Typical white LEDs have a forward voltage drop,
V
FWD_LED
, of 3.5V to 3.7V. Like single junction devices,
white LEDs often have poorly matched forward volt-
ages. If the LEDs were put in parallel without a series
resistor, the current in the paralleled branches would
vary, resulting in non-uniform brightness. R
LED,
in addi-
tion to setting the current, compensates for this varia-
tion by functioning as a ballast resistor, providing
negative feedback for each paralleled LED.
CM9156A Operation
When a voltage exceeding the undervoltage lockout
threshold (UVLO) is applied to the VIN pin, the
CM9156A initiates a softstart cycle, typically lasting
1000μs. Softstart limits the inrush current while the out-
put capacitors are charged during the power-up of the
device.
©
2006 California Micro Devices Corp. All rights reserved.
8
490 N. McCarthy Blvd., Milpitas, CA 95035-5112
l
Tel: 408.263.3214
l
Fax: 408.263.7846
l
www.cmd.com
04/26/06
CALMIRCO [ CALIFORNIA MICRO DEVICES CORP ]
