Capacitor type charge pumps are a well-known, simple, efficient, cost-effective (and therefore popular!) method for inverting and multiplying voltage supply rails. Perhaps less well known, however, is that they also work just as well for dividing voltage (while multiplying current). Figure 1 illustrates a V_{OUT} = V_{IN}/2, I_{OUT} = I_{IN}·2 example pump built around the venerable xx4053 CMOS triple SPDT switch.

Figure 1. |
xx4053 based, 100 kHz, voltage-halving, current-doublingcharge pump. |

Here’s how it works.

The R1C1 time constant couples the V_{IN}/ppv square wave found at U1pin14 to U1pin9, creating an F_{PUMP} oscillator frequency of (approximately):

During the F_{PUMP} negative half-cycle (U1pin4 = 0), the upper (U1pin14) end of C2 is connected to V_{IN} while the lower end (U1pin15) end is connected to V_{OUT}, thus charging C2 to:

Then, during the following F_{PUMP }positive half-cycle (U1pin4 = V_{IN}), the upper end of C2 connects to V_{IN} while the lower end connects to V_{OUT}, and:

This deposits a quantity of charge onto C3 of:

During the subsequent negative half-cycle, again:

Depositing another charge onto C3 of:

Thus, each full cycle of F_{PUMP} deposits a net charge onto C3 of:

Which, if I_{OUT} = 0, forces Q = 0 and therefore:

However, for the (much more interesting) case of I_{OUT} > 0:

In other words, V_{OUT} droops a bit as the output is loaded. This is because, for a finite C2 Q is also finite, but also to the fact that the U1a and U1b internal switches have non-zero ON resistances.

The combined effect on V_{OUT} versus I_{OUT} amounts to an effective impedance of 150 Ω for V_{IN} = 5 V and is plotted in Figure 2, along with current multiplication “efficiency”. Note that the latter soars past unity due to the fact that only half of the dollops of C2 charge (the Q_{+}) are drawn from the V_{IN} rail, while the Q_{–} are supplied from residual voltage on C2, causing zero additional drain from the rail.

Figure 2. |
Current multiplying charge-pump V_{OUT} and I_{OUT}/I_{IN }current “efficiency”for V _{IN} = 5 V. |

So, what is it good for?

Figure 3 suggests one useful application, generating nominally symmetrical ±V_{IN}/2 bipolar rails from a single positive source with minimal current draw from the source.

Figure 3. |
Current doubling charge pump plus voltage inverter makesan efficient bipolar rail splitter. |