Charge pump converts V IN to±V OUT
Ioan Ciasci, Rei Data, Cluj-Napoca, Romania -- EDN, 9/2/1999
A charge-pump IC's ability to produce both V
IN and -V
IN outputs allows the circuit in
Figure 1 to generate separate positive and negative outputs from a single input voltage, regardless of the input-voltage polarity. For example, the circuit allows an RS-232C interface to generate a dual supply for low-power data-acquisition systems (if you use TxD as V
IN, and you limit V
IN to the ±6V maximum that IC
1 can accommodate). Consider a positive V
IN: The positive V
OUT appears via D
1 and supplies power to the IC via Pin 8. D
2 and R
1 pull Pin 7 high, which is a condition (along with other connections) for setting the chip in its inverter mode. As a result, the circuit produces -V
OUT at Pin 5.
For a negative VIN, -VOUT appears via D3. D2 and R1 pull Pin 7 to ground, which, along with the Pin-5-to-Pin-6 connection, is a condition for setting the chip to its doubler mode. Thus, doubling -VIN produces VOUT at Pin 8. The MAX860 (or MAX861) operates at its minimum frequency of 6 kHz (13 kHz for the MAX861) with Pin 1 unconnected and at its maximum frequency of 130 kHz (250 kHz for the MAX861) with Pin 1 connected to Pin 5. To reduce the voltage drops associated with D1 and D3, you can replace the two Schottky diodes with MOSFETs. (DI #2402).
