AN857 Datasheet, PDF (2/6 Pages) Silicon Laboratories – SINGLE-CELL REGULATED Q-PUMP DRAWS LOW QUIESCENT CURRENT

AN857
Figure 1. Basic Low-Voltage Regulated Charge Pump
The TS12011 analog building-block IC from Silicon Labs is the heart of this device. It sips very low supply currents
(3.2 µA typ) and operates well down at the sub-1 V level necessary for single-cell operation. The comparator
output stage has good drive current capability down below 0.8 V VDD, an uncommon feature that allows the first
stage to be driven directly from the oscillator. The SN74AUP family logic gates are similarly super-low-power. The
MOSFETs were also carefully chosen for low-voltage operation, with low gate threshold voltage specs and low
gate-charge characteristics for low switching losses. Using these high-performance components, the no-load
quiescent current is a miserly 8 µA.
The RC oscillator is stopped when the output is in regulation via its LHDET latch input. When the output is above
the regulation threshold set by the reference and feedback divider, the op-amp output drives the latch input low,
latching the comparator output in the high state and stopping the oscillator. A large hysteresis band was chosen for
the oscillator, resulting in a large signal swing on the timing capacitor, C6. This achieves the most efficient size vs
current operating point for the oscillator. The maximum frequency is nominally set at 1 kHz with the component
values shown but can be adjusted up to about 3 kHz where it becomes limited by the propagation delay through
the comparator.
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Rev. 1.0