LT1054_15 Datasheet, PDF (12/33 Pages) Linear Technology – Switched-Capacitor Voltage Converter with Regulator

LT1054
SLVS033G – FEBRUARY 1990 – REVISED JULY 2015
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Device Functional Modes (continued)
A new variable, R_EQUIV, is defined as R_EQUIV = 1 / (f × C1). The equivalent circuit for the switched-capacitor
network is shown in Figure 14. The LT1054 has the same switching action as the basic switched-capacitor
building block. Even though this simplification does not include finite switch-on resistance and output-voltage
ripple, it provides an insight into how the device operates.
R_EQUIV
V1
V2
R_EQUIV
1
fC1
C2
R_L
Figure 14. Switched-Capacitor Equivalent Circuit
These simplified circuits explain voltage loss as a function of oscillator frequency (see Figure 7). As oscillator
frequency is decreased, the output impedance eventually is dominated by the 1 / (f × C1) term, and voltage
losses rise.
Voltage losses also rise as oscillator frequency increases. This is caused by internal switching losses that occur
due to some finite charge being lost on each switching cycle. This charge loss per-unit-cycle, when multiplied by
the switching frequency, becomes a current loss. At high frequency, this loss becomes significant and voltage
losses again rise.
The oscillator of the LT1054 is designed to operate in the frequency band where voltage losses are at a
minimum.
7.4.2 Shutdown
LT1054 can be put into a low quiescent current state by grounding the FB/SD pin. Once FB/SD is pulled low,
current being drawn from the supply will be approximately 100 µA.
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