Low-Noise, Dual-Output, Regulated Charge Pump

for GaAsFET, LCD, and VCO Supplies

_______________________________________________________________________________________

7

_______________Detailed Description

The MAX768 requires only seven external capacitors to

implement a regulated voltage doubler/inverter. These

can be ceramic or polarized electrolytic capacitors rang-

ing from 2.2µF to 47µF. Figure 1 is a functional diagram of

then stored in the V+ reservoir capacitor. Next, the volt-

voir capacitor. The voltages at V+ and V- are then linear

regulated and appear at POUT and NOUT, respectively.

The ripple noise induced by the doubling and inverting

charge pump is reduced by the linear regulators to

1.2mVp-p for POUT and 1.7mVp-p for NOUT. In addition,

the linear regulator’s excellent AC rejection attenuates

noise from the incoming supply. A minimum of 5mA is

available at each output. When NOUT is more negative

than 90% of the regulated output voltage, the open-drain

RDY output pulls to GND.

The charge pump operates in three modes: when

SYNC

= GND, the charge pump operates at 25kHz; when

SYNC = IN, it operates at 100kHz, or SYNC can be over-

driven with an external clock in the 20kHz to 240kHz

range. The clock must have a 40% to 60% duty cycle.

__________Applications Information

Setting the Output Voltage

Connect SETP or SETN directly to GND to select a fixed

+5V or -5V output voltage, respectively (Figure 2).

Select an alternative voltage for either output by con-

necting SETP or SETN to the midpoint of a resistor volt-

age divider from POUT or NOUT, respectively, to GND

value of the output voltage to ensure proper regulation.

Calculate the output voltage from the formulas below.

Choose R1 and R3 at between 100k

Ω to 400kΩ.

R2 = R1

R4 = R3

()

−









()

−









V

V

V

V

POUT

PSET REF

NOUT

NSET REF

1

1

MAX768

VOLTAGE

DOUBLER

+1.25V

REF

-1.25V

SHDN

POUT

SETP

CONNECT TO GND

CONNECT TO GND

GND

RDY

SETN

NOUT

IN

V+

C1+

C1-

PSHDN

SYNC

NSHDN

C2+

C2-

N

N

P

V-

VOLTAGE

INVERTER

Figure 1. Functional Diagram