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MAX864

Dual-Output Charge Pump with Shutdown

Maxim Integrated 

Dual-Output Charge Pump with Shutdown

Charge-Pump Output

The MAX864 is not a voltage regulator: the output

source resistance of either charge pump is approxi-

mately 55Ω at room temperature (with VIN = 5V); and V+

and V- 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- increases, since V-

is derived from V+. Treating each converter separately,

the droop of the negative supply (VDROOP-) is the prod-

uct of the current draw from V- (IV-) and the source

resistance of the negative converter (RS-):

VDROOP- = IV- x RS -

The droop of the positive supply (VDROOP+) is the

product of the current draw from the positive supply

(ILOAD+) and the source resistance of the positive con-

verter (RS+), where ILOAD+ is the combination of IV-

and the external load on V+ (IV+):

( ) VDROOP+ = ILOAD+ x RS+ = IV+ + IV- x RS+

Determine V+ and V- as follows:

V+ = 2VIN - VDROOP+

V - = (V+ - VDROOP )

= -(2VIN - VDROOP+ - VDROOP- )

The output resistances for the positive and negative

charge pumps are tested and specified separately. The

positive charge pump is tested with V- unloaded. The

negative charge pump is tested with V+ supplied from

an external source, isolating the negative charge pump.

Current draw from either V+ or V- is supplied by the

reservoir capacitor alone during one half cycle of the

clock. Calculate the resulting ripple voltage on either

output as follows:

VRIPPLE =

1

2

ILOAD

(1

/

fPUMP )

(1

/

CRESERVOIR )

where ILOAD is the load on either V+ or V-. For exam-

ple, with an fPUMP of 33kHz and 6.8µF reservoir capaci-

tors, the ripple is 26mV when ILOAD is 12mA.

Remember that, in most applications, the total load on

V+ is the V+ load current (IV+) and the current taken by

the negative charge pump (IV-).

Shutdown

The MAX864 features a shutdown mode that reduces

the maximum supply current to 1µA over temperature.

The SHDN pin is an active-low TTL logic-level input. If

the shutdown feature is unused, connect SHDN to IN.

In shutdown mode, V+ connects to IN through a 22Ω

switch and V- connects to GND through a 6Ω switch.

_________Efficiency Considerations

Theoretically, a charge-pump voltage multiplier can

approach 100% efficiency under the following condi-

tions:

• The charge-pump switches have virtually no offset,

and extremely low on-resistance.

• The drive circuitry consumes minimal power.

• The impedances of the reservoir and pump capaci-

tors are negligible.

For the MAX864, the energy loss per clock cycle is the

sum of the energy loss in the positive and negative

converters, as follows:

LOSSCYCLE = LOSSPOS + LOSSNEG

=

( ) 1

2

C1





V

+

2

−

2(V +) (VIN)

+

( ) 1 C2

2





V+

2

−

(V

−)2





where V+ and V- are the actual measured output volt-

ages.

The average power loss is simply:

Resulting in an efficiency of:

PLOSS = LOSSCYCLE x fPUMP

( ) η = Total Output Power / Total Output Power − PLOSS

There will be a substantial voltage difference between

(V+ - VIN) and VIN for the positive pump, and between

V+ and V- if the impedances of the pump capacitors

(C1 and C2) are large with respect to their respective

output loads.

Larger reservoir capacitor (C3 and C4) values will

reduce output ripple. Larger values of both pump and

reservoir capacitors will improve efficiency.

_______________________________________________________________________________________ 7

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