Low-Noise, Regulated, Negative

Charge-Pump Power Supplies for GaAsFET Bias

8

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___________________Chip Topography

________________________________________________________Package Information

sensitivity and interference. The clock must be a square wave

between 40% and 60% duty cycle. The maximum clock fre-

quency is 250kHz, and the minimum frequency is 50kHz.

Layout and Grounding

Good layout is important, primarily for good noise perfor-

mance.

1) Mount all components as close together as possible.

2) Keep traces short to minimize parasitic inductance

and capacitance. This includes connections to FB.

3) Use a ground plane.

Noise and Ripple Measurement

Accurately measuring the output noise and ripple is a chal-

lenge. Brief differences in ground potential between the

MAX850–MAX853 circuit and the oscilloscope (which result

from the charge pump’s switching action) cause ground

currents in the probe’s wires, inducing sharp voltage spikes.

For best results, measure directly across the output capaci-

tor (C4). Do not use the ground lead of the oscilloscope

probe; instead, remove the probe’s tip cover and touch the

ground ring on the probe directly to C4’s ground terminal.

You can also use a Tektronix chassis mount test jack (part

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

direct connection gives the most accurate noise and rip-

ple measurement.

FB (MAX850–852)

CONT (MAX853)

OUT

GND

IN

NEGOUT

C1-

C1+

SHDN (MAX850/853)

SHDN (MAX851)

OSC (MAX852)

0.127"

(3.226mm)

0.085"

(2.159mm)

TRANSISTOR COUNT: 164

SUBSTRATE CONNECTED TO IN

L

DIM

A

A1

B

C

D

E

e

H

h

L

α

MIN

0.053

0.004

0.014

0.007

0.189

0.150

0.228

0.010

0.016

0˚

MAX

0.069

0.010

0.019

0.010

0.197

0.157

0.244

0.020

0.050

8˚

MIN

1.35

0.10

0.35

0.19

4.80

3.80

5.80

0.25

0.40

0˚

MAX

1.75

0.25

0.49

0.25

5.00

4.00

6.20

0.50

1.27

8˚

INCHES

MILLIMETERS

α

8-PIN PLASTIC

SMALL-OUTLINE

PACKAGE

H

E

D

e

A

A1

C

h x 45˚

0.127mm

0.004in.

B

1.27 BSC

0.050 BSC

21-325A