Compact, Dual-Output Charge Pump

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5

_______________Detailed Description

The MAX865 contains all the circuitry needed to imple-

ment a voltage doubler/inverter. Only four external

capacitors are needed. These may be polarized elec-

trolytic or ceramic capacitors with values ranging from

1µF to 100µF.

Figure 2a shows the ideal operation of the positive volt-

age doubler. The on-chip oscillator generates a 50%

duty-cycle clock signal. During the first half cycle,

switches S2 and S4 open, switches S1 and S3 close,

During the second half cycle, switches S1 and S3

open, switches S2 and S4 close, and capacitor C1 is

and no load on C3, charge transfers into C3 from C1

positive supply output (V+).

Figure 2b illustrates the ideal operation of the negative

converter. The switches of the negative converter are

out of phase with the positive converter. During the

second half cycle, switches S6 and S8 open and

switches S5 and S7 close, charging C2 from V+

GND. In the first half of the clock cycle, switches S5

and S7 open, switches S6 and S8 close, and the

charge on capacitor C2 transfers to C4, generating the

negative supply. The eight switches are CMOS power

MOSFETs. Switches S1, S2, S4, and S5 are P-channel

devices, while switches S3, S6, S7, and S8 are N-chan-

nel devices.

Charge-Pump Output

The MAX865 is not a voltage regulator: the output

source resistance of either charge pump is approxi-

mately 150

V+ and V- will approach +10V and -10V, respectively,

when lightly loaded. Both V+ and V- will droop toward

GND as the current draw from either V+ or V- increas-

es, since V- is derived from V+. Treating each convert-

er separately, the droop of the negative supply

er (RS-):

product of the current draw from the positive supply

IN

a)

b)

S1

S3

C1+

C1

C3

C1-

S2

S4

S5

S6

S7

S8

C2-

GND

V-

C2+

C4

C2

GND

GND

V+

V+

Figure 2. Idealized Voltage Quadrupler: a) Positive Charge Pump; b) Negative Charge Pump

V

= I

x RS -

DROOP -

V -