LT1054_15 Datasheet, PDF (23/33 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

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LT1054
SLVS033G – FEBRUARY 1990 – REVISED JULY 2015
9 Power Supply Recommendations
The LT1054 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 will be approximately equal to 2.2 times
the output current. During the time that C_IN is delivering charge to C_OUT the supply current drops to approximately
0.2 times the output current. An input supply bypass capacitor will supply part of the peak input current drawn by
the LT1054 and average out the current drawn from the supply. A minimum input supply bypass capacitor of
2µF, preferably tantalum or some other low ESR type is recommended. A larger capacitor may be desirable in
some cases, for example, when the actual input supply is connected to the LT1054 through long leads, or when
the pulse current drawn by the LT1054 might affect other circuitry 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 30). 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.
( ) ^V_OUT ^b
R1 £
I_OUT
(11)
V_IN
1
FB/SD
8
V_CC
Load
2
CAP+
7
OSC
V_OUT
R1
LT1054
C_IN ⁺
3
GND
6
V_REF
4
CAP−
5
V_OUT
+ ^COUT
Figure 30. Circuit With Load Connected from V_CC to V_OUT
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