Low-Noise, Dual-Output, Regulated Charge Pump

for GaAsFET, LCD, and VCO Supplies

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9

Switching-Frequency Control

For applications sensitive to the MAX768’s internal

switching frequency, connect an external TTL/CMOS

(within IN and GND) clock to SYNC. The clock must be

a 20kHz to 240kHz square wave between 40% and

60% duty cycle.

Schottky Diodes

When under heavy loads, where POUT is sourcing into

NOUT (i.e., load current flows from POUT to NOUT,

rather than from supply to ground), do not allow NOUT

to pull above ground. In applications where large cur-

rents from POUT to NOUT are likely, use a Schottky

diode (1N5817) between GND and NOUT, with the

anode connected to GND (Figure 4).

Connect a IN5817-type Schottky diode from C2- to V-

to assure proper start-up.

Layout and Grounding

Good layout is important, primarily for good noise per-

formance:

1) Mount all components as close together as possi-

ble.

2) Keep traces short to minimize parasitic inductance

and capacitance. This includes connections to

SETP and SETN.

3) Use a ground plane.

Noise and Ripple Measurement

Accurately measuring output noise and ripple can be

difficult. Brief differences in ground potential between

the circuit and the oscilloscope (which result from the

charge pump’s switching action) cause ground cur-

rents in the probe’s wires, inducing sharp voltage

spikes. For best results, measure directly across output

capacitor C3, C4, C5, or C6. Do not use the oscillo-

scope probe’s ground lead; instead, remove the

cover’s ground lead and touch the ground ring on the

probe directly to the ground terminal of C3, C4, C5, or

C6. Or, use a Tektronix chassis-mount test jack (part

no. 131-0258) to connect your scope probe directly.

This direct connection provides the most accurate

noise and ripple measurement.

MAX768

IN

SYNC

C2

C1

C1+

C1-

C2+

C3

C5

C6

R2

R1

R3

R4

R4

R3

C4

)

C2-

NSHDN

PSHDN

POUT

SETP

V+

GND

V-

SETN

NOUT

RDY

R2

R1

)

Figure 3. MAX768 Adjustable Configuration

MAX768

GND

NOUT

Figure 4. A Schottky diode protects the MAX768 when a large

current flows from POUT to NOUT.