AN857 Datasheet, PDF (3/6 Pages) Silicon Laboratories – SINGLE-CELL REGULATED Q-PUMP DRAWS LOW QUIESCENT CURRENT

AN857
Figure 2. TS12011 Charge-Pump Waveforms:
Vout (top), Comparator Output (middle), and Op-Amp Output (bottom)
The amount of charge transferred with each cycle and the switching frequency determine the output current.
Making an accurate calculation for the output impedance or output current from a regulated two-stage pump is
complex and needs a large spreadsheet to show. Just to get into the right ballpark, it is possible to make some
gross oversimplifications. Assuming the capacitor is completely charged and discharged with each cycle (which
obviously isn’t true), the following can be written:
Q = C  V and I = Q  f so I = C  V  F
Equation 1.
Where C is the flying capacitor value; V is the applied voltage during the charging phase; F is the oscillator
frequency, and I is the output current if all the stored charge during the discharge phase (roughly equivalent to a
short-circuit load current) could be removed.
The modified Dickson Multiplier 2-stage pump topology employed here is a quadrupler, and both stages must be
designed to transfer the necessary charge at the worst-case low-battery voltage. The first stage is the most
important because the voltage applied to the capacitor is low. Any drop across the first rectifier subtracts from the
applied voltage with a resulting loss in headroom, which causes difficulty when trying to multiply up to 2.5 V output
levels. The rectifier choice is further complicated in this low-quiescent application by leakage currents, which
effectively load down the pump and increase idling current. The BAS52-02V Schottky diode selected has a good
blend of low reverse leakage, low forward drop, small packaging, and wide availability. Reverse leakage current at
high temperature is the Achilles heel of Schottky diodes, but lab measurements show that the typical BAS52 works
well in this regard (less than 1 uA at 50 °C). For even lower reverse leakage, there is the BAS40-02V, but the trade-
off is about 75 mV higher forward drop.
Rev. 1.0
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