Low-Noise, Regulated, -2V GaAsFET Bias

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_______________Detailed Description

The MAX840/MAX843/MAX844 are low-noise, inverting,

regulated charge-pump power supplies designed for

biasing GaAsFET devices, such as power-amplifier

modules in cellular handsets.

ative voltage at NEGOUT by a capacitive charge pump.

This voltage is then regulated by an internal low-noise

linear regulator, and appears at OUT (Figure 1). The

minimum (most negative) output voltage achievable is

the inverted positive voltage, plus the 0.6V required by

the post-regulator. For the MAX840, the linear regulator

reduces ripple noise induced by the charge-pump

lator’s excellent AC rejection attenuates noise from the

incoming supply.

__________Applications Information

Setting the Output Voltage

For the MAX840, select either a fixed or adjustable

output voltage. Connect FB directly to GND for a fixed

-2V output (Figure 2a). Select an alternate output volt-

age by connecting FB to the midpoint of a resistor

allow proper regulation. The output voltage is calculat-

ed from the formula below. Choose R2 to be between

100k

Ω and 400kΩ.

For the MAX843/MAX844, set the output voltage by

connecting a resistor voltage divider between OUT and

Shutdown Mode

The MAX840/MAX843/MAX844 feature a shutdown

mode that reduces the supply current to 1µA max over

temperature (300µA max for the MAX844). When the

MAX840/MAX843 are in shutdown, the outputs (OUT,

NEGOUT) and the charge-pump oscillator are dis-

abled. When the MAX844 is in shutdown, only the lin-

ear regulator is disabled and the NEGOUT output

remains enabled. However, the charge-pump oscilla-

tion frequency is reduced to 20kHz, reducing the

available power at NEGOUT. The output voltage at

NEGOUT can be used to bias an LCD while in shut-

down.

Capacitors

Use capacitors with low effective series resistance

The overall 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 frequency, 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.

The output resistance of the entire circuit is approxi-

mately:

of the internal switches and the resistance across the

linear regulator, is approximately 71

48

C1, C2, and C3 should be 1µF capacitors with less than

0.8

Ω ESR. C4 should be a 10µF capacitor with less

than 0.2

Ω ESR. Smaller capacitor values can be used

(C1 = C2 = C3 = 0.22µF, C4 = 4.7µF) with a small

increase in output noise and ripple (Figure 2d). All

capacitors should be either surface-mount chip tanta-

lum or ceramic types. External capacitor values can be

adjusted to optimize size and cost.

Layout and Grounding

Good layout is important, primarily for good noise perfor-

mance. Take the following steps to ensure good layout:

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

challenge. Slight momentary differences in ground

potential between the MAX840/MAX843/MAX844 circuit

and the oscilloscope (which results 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 capacitor

(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 ripple measurement.