CM9130 Datasheet, PDF (7/12 Pages) California Micro Devices Corp – Three Output Driver for White LEDs

PRELIMINARY
CM9130
Application Information (cont’d)
The 1.5x mode employs a fractional charge pump. The
charge pump uses two phases from the internal oscil-
lator 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 connected in
series and each are charged from V_IN 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 VIN. 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
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
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 CM9130 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
The CM9130 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
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Many charge pumps are fixed 2x designs. The ideal 2x
charge pump efficiency can be similarly expressed;
P----_O----U----T-
P_IN
≈
2---.--0-3---.×^{--9---VV-----}_I^--_N^--
In 1x mode, when the input voltage is above the output
voltage, the ideal efficiency is simply V_OUT/V_IN.
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