CM9156B Datasheet, PDF (8/13 Pages) California Micro Devices Corp – Charge-Pump White LED Driver

PRELIMINARY
CM9156B
Application Information
The CM9156B 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 CM9156B charge pump is the perfect driver for
such portable applications, providing efficiency, com-
pact overall size, low system cost and minimum EMI.
The CM9156B 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 CM9156B 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 CM9156B 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
=
4.5V
− V_{FWD _ LED}
R_LED
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.
CM9156B Operation
When a voltage exceeding the undervoltage lockout
threshold (UVLO) is applied to the VIN pin, the
CM9156B 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.
The input voltage, VIN, passes through an LDO pre-
regulator that compares the output voltage to a preci-
sion 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 Figure 1. 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.
VIN
FB
LDO
V_LDO
VOUT
C1
½ V_LDO
C _{O UT}
C2
½ V_LDO
Charge C1 and C2 to ½ V _LDO
VOUT
VIN
FB
LDO
V_LDO
C1
½ V_LDO
C _{O UT}
C2
½ V_LDO
Figure 1. Switch operation
© 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