Low-Noise, Regulated, -2V GaAsFET Bias
MAX840/MAX843/MAX844
_______________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.
The applied input voltage (V
IN
) is first inverted to a neg-
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
inverter to 1mVp-p at V
OUT
. In addition, the linear regu-
lator’s excellent AC rejection attenuates noise from the
incoming supply.
Capacitors
Use capacitors with low effective series resistance
(ESR) to maintain a low dropout voltage (V
IN
-
|
V
OUT
|
).
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:
R
OUT
= R
O
+ 4 x ESR
C1
+ ESR
C4
+
1 / (f
S
x C1) + R
(linear regulator)
where [R
O
+ R
(linear regulator)
], the effective resistance
of the internal switches and the resistance across the
linear regulator, is approximately 71Ω at V
IN
= 2.5V,
48Ω at V
IN
= 5V, and 40Ω at V
IN
= 10V.
C1, C2, and C3 should be 1µF capacitors with less than
0.8Ω ESR. C4 should be a 10µF capacitor with less
than 0.2Ω 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.
__________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
voltage divider from OUT to GND (Figure 2b). V
IN
must be 0.6V above the absolute value of V
OUT
to
allow proper regulation. The output voltage is calculat-
ed from the formula below. Choose R2 to be between
100kΩ and 400kΩ.
V
OUT
= (-0.5V)(1 + R2 / R1)
For the MAX843/MAX844, set the output voltage by
connecting a resistor voltage divider between OUT and
a positive control voltage (V
CTRL
) (Figure 2c).
V
OUT
= -V
CTRL
(R2 / R1)
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.
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.
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.
8
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