A substantial voltage difference exists between (V+ -

V+

and

V- if the impedances of the pump capacitors

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

loads.

Larger values of reservoir capacitors (C3 and C4)

reduce output ripple. Larger values of both pump and

reservoir capacitors improve power efficiency.

Charge-Pump Capacitor Selection

To maintain the lowest output resistance, use capacitors

with low effective series resistance (ESR). The charge-

pump output resistance is a function of C1, C2, C3, and

C4’s ESR. Therefore, minimizing the charge-pump

capacitors’ ESR minimizes the total output resistance.

__________Applications Information

Positive and Negative Converter

The MAX865 is most commonly used as a dual charge-

pump voltage converter that provides positive and neg-

ative outputs of two times a positive input voltage. The

Typical Operating Circuit shows that only four external

components are needed: capacitors C1 and C3 for the

positive pump, C2 and C4 for the negative pump. In

most applications, all four capacitors are low-cost,

3.3µF polarized electrolytics. For applications where PC

board space is at a premium and very low currents are

being drawn from the MAX865, 1µF capacitors may be

used for the pump capacitors C1 and C2, with 1µF

reservoir capacitors C3 and C4. Capacitors C2 and C4

must be rated at 12V or greater.

Paralleling Devices

Paralleling multiple MAX865s (Figure 3) reduces the

output resistance of both the positive and negative con-

verters. The effective output resistance is the output

resistance of one device divided by the number of

devices. Separate C1 and C2 charge-pump capacitors

are required for each MAX865, but the reservoir capac-

itors C3 and C4 can be shared.

Heavy Output Current Loads

When under heavy loads, where V+ is sourcing current

into V- (i.e., load current flows from V+ to V-, rather than

from supply to ground), do not allow the V- supply to

pull above ground. In applications where large currents

flow from V+ to V-, use a Schottky diode (1N5817)

between GND and V-, with the anode connected to

GND (Figure 4).

Layout and Grounding

Good layout is important, primarily for good noise per-

formance. To ensure good layout:

• Mount all components as close together as possible

• Keep traces short to minimize parasitic inductance

and capacitance

• Use a ground plane.

Compact, Dual-Output Charge Pump

_______________________________________________________________________________________

7

MAX865

GND

V-

Figure 4. A Schottky diode protects the MAX865 when large

currents flow from V+ to V-.