CM9142 Datasheet, PDF (7/14 Pages) California Micro Devices Corp – Two Group, 4 and 2, WLED Driver, Different Current Settings

PRELIMINARY
CM9142
Application Information (cont’d)
tional charge pump to convert the nominal Li-ion bat-
tery voltage (3.6V) by 1.5 times and regulates the LED
current to the low dropout current sources.
The current regulated sources maintain constant LED
drive in the presence of supply voltage fluctuations. All
LEDs are driven with the same current, even when they
have slightly different forward voltages. The individual
current sources sense the current through each LED
and match this current to less than 2% for uniform
brightness across the color LCD display.
The CM9142 drives up to four WLEDS in group one
and two WLEDs in the second group. The maximum
current programmed by R_SET determines the maxi-
mum intensity of each group’s display; the displays can
be further dimmed by PWM control applied to its ISET1
or ISET 2 pin.
CM9142 Operation
When a voltage that exceeds the undervoltage lockout
threshold (UVLO) is applied to the VIN pin, the
CM9142 initiates a softstart cycle, typically lasting
100μS. Softstart limits the inrush current while the
output capacitors are charged. Following softstart, the
CM9142 next determines the best conversion ratio (1x
or 1.5x).
The 1.5x mode employs a fractional charge pump. The
charge pump uses two phases from the internal oscilla-
tor to drive 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 Vin to a
voltage of V_IN/2. The next phase changes the switch
positions so that C1 and C2 are in parallel, and places
them on top of V_IN. The resulting voltage across C_OUT
is then; V_IN+1/2V_IN = 1.5 x V_IN.
VIN
VOUT
C1
½ VIN
C_OUT
C2
½ VIN
Charge C1 and C2 to ½ VIN each
VOUT
VIN
C1
½ VIN
C_OUT
C2
½ VIN
Transfer ½ VIN charge to top of VIN
Figure 1. Switch Operation
The CM9142 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 400~600mA)
and will cause the output voltage to drop, until automat-
ically 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 approxi-
mately 15°C.
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
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