LT1054 Datasheet, PDF (9/26 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

LT1054
SWITCHEDĆCAPACITOR VOLTAGE CONVERTERS
WITH REGULATORS
SLVS033F − FEBRUARY 1990 − REVISED NOVEMBER 2004
PRINCIPLES OF OPERATION
Supply voltage V_CC alternately charges C_IN to the input voltage when C_IN is switched in parallel with the input supply
and then transfers charge to C_OUT when C_IN is switched in parallel with C_OUT. Switching occurs at the oscillator
frequency. During the time that C_IN is charging, the peak supply current is approximately 2.2 times the output current.
During the time that C_IN is delivering a charge to C_OUT, the supply current drops to approximately 0.2 times the output
current. An input supply bypass capacitor supplies part of the peak input current drawn by the LT1054 and averages
the current drawn from the supply. A minimum input-supply bypass capacitor of 2 µF, preferably tantalum or some
other low equivalent-series-resistance (ESR) type, is recommended. A larger capacitor is desirable in some cases.
An example of this would be when the actual input supply is connected to the LT1054 through long leads or when
the pulse currents drawn by the LT1054 might affect other circuits through supply coupling.
In addition to being the output terminal, V_OUT is tied to the substrate of the device. Special care must be taken in
LT1054 circuits to avoid making V_OUT positive with respect to any of the other terminals. For circuits with the output
load connected from V_CC to V_OUT or from some external positive supply voltage to V_OUT, an external transistor must
be added (see Figure 14). This transistor prevents V_OUT from being pulled above GND during startup. Any small
general-purpose transistor such as a 2N2222 or a 2N2219 device can be used. Resistor R1 should be chosen to
provide enough base drive to the external transistor so that it is saturated under nominal output voltage and maximum
output current conditions.
R1
v
ǒŤ_VOUT^ŤǓ_b
I_OUT
(4)
VIN
1
FB/SD
8
VCC
Load
2
CAP+
7
OSC
VOUT
R1
LT1054
CIN ⁺
3
GND
6
VREF
4
CAP−
5
VOUT
+ ^COUT
Pin numbers shown are for the P package.
Figure 14. Circuit With Load Connected from V_CC to V_OUT
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