Charge pump converts –5V to 5V
Edited by Brad Thompson
Ken Yang, Maxim Integrated Products Inc, Sunnyvale, CA -- EDN, January 20, 2005
Versatile switched-capacitor charge-pump voltage converters can provide a negative supply voltage from a positive-voltage source or double a positive source's voltage. However, certain applications that consist entirely of ECL (emitter-coupled-logic) circuits provide only a negative-voltage supply—for example, –5.2V. Figure 1 shows how you can use a switched-capacitor converter to obtain a positive power-supply voltage suitable for powering ECL-to-TTL (transistor-to-transistor-logic)-level translators and other circuits.
Although connections to IC1 may appear to be reversed, the bilateral characteristics of IC1's internal switches allow use of IC1's output pin as its power input. Capacitor C1 acquires a charge when IC1's internal switches connect the CAP+ pin to ground and CAP– to the negative-voltage power source via the output pin, OUT. During the next half-cycle, IC1 connects CAP– to ground and CAP+ to IN (normally used as the input), transferring C1's positive charge to output capacitor C3 and the load. With FSEL connected to OUT, an internal oscillator sets the charge-discharge cycle's frequency to approximately 1 MHz.
As Figure 2 shows, IC1's switches present internal resistances that affect the output voltage's magnitude, which is lower than the input voltage and subject to less-than-ideal regulation as output-load current increases. For optimum performance, use low-ESR capacitors for C1, and input and output bypass capacitors C2 and C3.
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