AN857 Datasheet, PDF (4/6 Pages) Silicon Laboratories – SINGLE-CELL REGULATED Q-PUMP DRAWS LOW QUIESCENT CURRENT

AN857
The capacitor values and switching frequency are chosen to be somewhat overkill for charge transfer so that they
don’t get in the way of the voltage drop/headroom issues. Thus, the frequency is low and the caps are relatively
large, which also keeps switching losses in check. More importantly, a long ON-time allows the voltage across the
diode to reach a minimum, with the tail end of the forward voltage vs. current curve being reached (a little less than
0.2 V for the BAS52).
From the simplified charge-transfer equations above, an overly-optimistic estimate for first-stage current, which
ought to be sufficient overkill for trying to hit a 50 µA load current requirement, is shown below:
VDD – VFWD – VSAT1  C3  F = 0.95 V – 0.2 V – 0.05 V  2.2 f  1 kHz = 1.5 mA
Equation 2.
The best-possible peak output voltage that can be achieved for the intermediate V1 tap:
2VDD – 2VFWD – VSAT1 (high) – VSAT1 (low)
Equation 3.
Similarly, the second-stage peak voltage for Vout:
2V1 – 2VFWD – VSAT2 (high) – VSAT2 (low)
Equation 4.
So, the best-possible peak load voltage at the minimum input voltage for Vout:
4VDD – 4VFWD – 4VSAT = 4  0.95 V – 4  0.2 V – 2  0.05 V – 2  0.1 V = 2.7 V
Equation 5.
Where VSAT1 and VSAT2 are the voltage drops across the comparator and the AUP inverter output stages in the
ON state, respectively. Again, this analysis is oversimplified but points out where the limits of operation will lie.
This math reveals that there is not a lot of room for temperature variation and component tolerances when trying to
quadruple the lower end of the voltage range of a single cell. We can improve this and get to sub-0.9 V operation,
by one of several means, such as: a) adding another pump stage; b) changing the output voltage to 2.2 V—
possibly a less interesting design challenge, or c) adding a synchronous rectifier across the first diode.
The third option, a synchronous rectifier (Figure 3), is chosen here due to its good efficiency at the sweet spots. A
low-threshold P-channel MOSFET having low gate charge shunts the first pump diode, reducing the forward drop
from 0.2 V to 0.01 V or so. This gives a couple hundred extra millivolts right at the front end, where it’s needed
most.
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Rev. 1.0