Low-Noise, Regulated, Negative

Charge-Pump Power Supplies for GaAsFET Bias

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7

__________Applications Information

Setting the Output Voltage

For the MAX850–MAX852, select either a fixed or an

adjustable output voltage. Connect FB directly to GND to

select the fixed -4.1V output (Figure 2a). To select an alter-

nate output voltage, connect FB to the midpoint of a resis-

regulation. The output voltage is calculated from the formu-

la below. Choose R2 to be between 100k

Ω to 400kΩ.

For the MAX853, set the output voltage, VOUT, by con-

necting a resistor voltage divider between OUT and a

Shutdown

The MAX850–MAX853 feature a shutdown mode that

reduces the supply current to 1µA max over temperature

(5µA max for the MAX851). The MAX850 and MAX853 have

an active-low TTL logic level SHDN input, whereas the

MAX851 has an active-high SHDN input. To shut down the

MAX852, set the OCSC input to a logic-low level. The device

is powered up by the resumption of the clock signal.

Capacitors

Use capacitors with low effective series resistance (ESR) to

dropout voltage is a function of the charge pump’s output

resistance and the voltage drop across the linear regulator

(N-channel pass transistor). At the 100kHz switching fre-

quency, the charge-pump output resistance is a function of

C1 and C2’s ESR. Therefore, minimizing the ESR of the

charge-pump capacitors minimizes the dropout voltage.

1µF, 0.8

Ω ESR capacitors are recommended for C1, C2,

and C3. C4 should be 10µF, 0.2

Ω ESR. All capacitors

should be either surface-mount chip tantalum or chip

ceramic types. External capacitor values may be adjusted to

optimize size and cost.

Switching-Frequency Control

Use the MAX852 to minimize system interference caused by

conflicting clock frequencies. An external oscillator can set

the charge-pump frequency and reduce clock frequency

MAX850

MAX851

MAX852

FB

OUT

SHDN*

SHDN

OSC

NEGOUT

C1-

C1+

IN

GND

C3

1

µF

*MAX850: SHDN

MAX851: SHDN

MAX852: OSC

C4

10

µF

C1

1

µF

C2

1

µF

MAX853

CONT

OUT

SHDN

NEGOUT

C1-

C1+

IN

GND

C3

1

µF

C4

10

µF

C1

1

µF

C2

1

µF

R2

100k

R1

100k

Figure 2a. MAX850/MAX851/MAX852 Standard Application Circuit

Figure 2c. MAX853 Standard Application Circuit

MAX850

MAX851

MAX852

FB

OUT

SHDN*

SHDN

OSC

NEGOUT

C1-

C1+

IN

GND

C3

1

µF

R2

R1

*MAX850: SHDN

MAX851: SHDN

MAX852: OSC

C4

10

µF

C1

1

µF

C2

1

µF

R2

100k

R1

100k

(

)

Figure 2b. MAX850/MAX851/MAX852 Adjustable Configuration

V

= -1.28

OUT









1

2

1

R

R

V

= - V

R2

R1

OUT

CTRL







