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General Description
The MAX881R low-noise, inverting power supply is
designed for biasing GaAsFET power amplifiers in
portable wireless applications. This device is a chargepump inverter followed by a negative linear regulator.
The input voltage range is 2.5V to 5.5V. The output is
preset at -2.0V or can be set, using two resistors, to any
voltage from -0.5V to (-VIN+ 0.6V). It can deliver up to
4mA. The internal linear regulator also filters the output
to 1mVp-p ripple and noise.
Other features include a power-OK (POK) output that signals when the negative voltage is within 7.5% of its set
point. It protects the GaAsFET by not allowing power to
be applied to the GaAsFET’s drain until it is properly
biased. The signal can be routed either to a microcontroller or directly to a switch at the GaAsFET drain. The
MAX881R is available in a space-saving 10-pin µMAX
package.
Applications
Cell Phones
PCS Phones
PHS Phones
Wireless Handsets
Wireless Modems
Two-Way Pagers
Mobile Radios
Wireless Computers
Features
♦ Small µMAX Package
♦ 1mVp-p Output Voltage Ripple and Noise
♦ Power-OK Signal to Control GaAsFET Drain
Switch
♦ 0.05µA Logic-Controlled Shutdown
♦ 1ms Guaranteed Start-Up
♦ 2.5V to 5.5V Input
♦ -0.5V to (-V
IN
+ 0.6V) Output at up to 4mA
♦ Operates with One 4.7µF and Three 0.22µF
Capacitors (no inductors needed)
MAX881R
Low-Noise Bias Supply in µMAX
with Power-OK for GaAsFET PA
________________________________________________________________
Maxim Integrated Products
1
1
2
3
4
5
10
9
8
7
6
IN
N.C.
GND
OUTPOK
N.C. = NOT INTERNALLY CONNECTED
NEGOUT
C1-
C1+
MAX881R
µMAX
TOP VIEW
FBSHDN
Typical Operating Circuit
19-1372; Rev 1; 6/98
PART
MAX881REUB -40°C to +85°C
TEMP. RANGE PIN-PACKAGE
10 µMAX
EVALUATION KIT
AVAILABLE
Pin Configuration
Ordering Information
ON
INPUT
2.5V TO 5.5V
OFF
OUTPUT
-2V (OR ADJ)
4mA, 1mVp-p
IN
MAX881R
SHDN
C1+
C1-
NEGOUT
FB
OUT
POK
GND
RIPPLE
CONTROLS
GaAsFET PA’s
DRAIN SWITCH
MAX881R
Low-Noise Bias Supply in µMAX
with Power-OK for GaAsFET PA
2 _______________________________________________________________________________________
ABSOLUTE MAXIMUM RATINGS
ELECTRICAL CHARACTERISTICS
(Circuit of Figure 3, VIN= +3.6V, FB = GND, RL= ∞, SHDN = IN, TA= -40°C to +85°C, unless otherwise noted. Typical values are at
T
A
= +25°C.) (Note 2)
Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional
operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to
absolute maximum rating conditions for extended periods may affect device reliability.
IN to GND.................................................................-0.3V to +6V
SHDN to GND...........................................................-0.3V to +6V
POK to GND...........................................................-0.3V to +12V
C1+ to GND.................................................-0.3V to (V
IN
+ 0.3V)
C1-, NEGOUT, OUT, FB to GND ....................-6V to (V
IN
+ 0.3V)
Continuous Power Dissipation (T
A
= +70°C)
10-Pin µMAX (derate 5.6mW/°C above +70°C)...........444mW
Operating Temperature Range ...........................-40°C to +85°C
Junction Temperature......................................................+150°C
Storage Temperature Range.............................-65°C to +165°C
Lead Temperature (soldering, 10sec).............................+300°C
VIN= 3V, RL= 500Ω, V
SHDN
= 0 to VIN,
POK goes low
VIN≥ 2.5V, I
OUT
= 0 to 4mA
Connected to IN or GND
VIN= 2.5V
V
IN
≥ 2.7V, I
OUT
= 0 to 4mA
VIN= 5.5V
V
POK
= 11V
I
OUT
= 4mA, circuit of Figure 3b
V
IN
≥ 2.7V, I
OUT
= 0 to 4mA
VIN≥ 2.5V, sinking 1mA
SHDN = GND
VFB= -0.5V
FB = OUT
CONDITIONS
ms1t
START
Start-Up Time
pF10C
IN
SHDN Input Capacitance
µA±1I
SHDN
SHDN Input Current
V0.35V
IL
SHDN Input Low Voltage
V2.2V
IH
SHDN Input High Voltage
µA1
POK Off Leakage Current
mV100
POK Output Level
% of
V
OUT
90 92.5 95
POK Threshold
kHz80 100 120f
OSC
Oscillator Frequency
V-(VIN- 0.6) -0.5V
OUT
Adjustable Output Voltage
Range
V-2.1 -2.0 -1.9V
OUT
V2.5 5.5V
IN
Supply Voltage Range
Preset Output Voltage
mVp-p1Output Ripple
mV/mA2 6Output Load Regulation
µA0.05 1I
SHUT
Shutdown Supply Current
V
-0.515 -0.5 -0.485
V
FB
FB Voltage
nA-10 -100FB Input Current
µA500 950I
Q
Supply Current
UNITSMIN TYP MAXSYMBOLPARAMETER
Note 1: The output may be shorted to NEGOUT or GND if the package power dissipation is not exceeded. Typical short-circuit
current is 35mA.
-0.525 -0.475
-0.535 -0.465
VIN≥ 2.5V,
I
OUT
= 0 to 4mA
TA= +25°C
TA= 0°C to +85°C
TA= -40°C to +85°C
Note 2: Specifications to -40°C are guaranteed by design, not production tested.
MAX881R
Low-Noise Bias Supply in µMAX
with Power-OK for GaAsFET PA
_______________________________________________________________________________________
3
-2.02
-1.98
-2.00
-1.94
-1.96
-1.92
1.90
-1.86
-1.88
-1.84
0 2 3 41 5 6 7 8 9 10
OUTPUT VOLTAGE
vs. OUTPUT CURRENT
MAX881R TOC01
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
CIRCUIT OF FIGURE 3a
V
IN
= 2.5V
V
IN ≥
3.6V
-2.02
-1.98
-2.00
-1.92
-1.94
-1.96
-1.86
-1.88
-1.90
-1.84
0 6 82 4 10 12 14 16
OUTPUT VOLTAGE
vs. OUTPUT CURRENT
MAX881R TOC02
OUTPUT CURRENT (mA)
OUTPUT VOLTAGE (V)
CIRCUIT OF FIGURE 3b
V
IN
= 2.5V
V
IN ≥
3.6V
-2.03
-2.01
-2.02
-1.99
-2.00
-1.98
-1.97
-1.96
-1.95
2.0 3.0 3.52.5 4.0 4.5 5.0 5.5 6.0
OUTPUT VOLTAGE
vs. INPUT VOLTAGE
MAX881R TOC03
INPUT VOLTAGE (V)
OUTPUT VOLTAGE (V)
I
LOAD
= 4mA
Typical Operating Characteristics
(Circuit of Figure 3, VIN= 3.6V, TA= +25°C, unless otherwise noted.)
0
10
5
25
20
15
40
35
30
45
2.0 3.5 4.02.5 3.0 4.5 5.0 5.5 6.0
MAXIMUM OUTPUT CURRENT
vs. INPUT VOLTAGE
MAX881r TOC04
INPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
V
OUT
< 2mVp-p RIPPLE
CIRCUIT OF FIGURE 3b
V
OUT
< 2mVp-p RIPPLE
CIRCUIT OF FIGURE 3a
V
OUT
≤ -1.8V
CIRCUIT OF FIGURE 3a
V
OUT
≤ -1.8V
CIRCUIT OF FIGURE 3b
0
4
8
12
16
20
24
2.0 3.02.5 3.5 4.0 4.5 5.0 5.5 6.0
MAXIMUM NEGOUT CURRENT
vs. INPUT VOLTAGE
MAX881R TOC05
INPUT VOLTAGE (V)
OUTPUT CURRENT (mA)
V
NEGOUT
= 0.9 x V
NEGOUT(NO LOAD)
CIRCUIT OF FIGURE 3b
CIRCUIT OF FIGURE 3a
300
400
500
600
700
800
900
2.0 3.02.5 3.5 4.0 4.5 5.0 5.5 6.0
NO-LOAD SUPPLY CURRENT
vs. INPUT VOLTAGE
MAX881R TOC06
INPUT VOLTAGE (V)
SUPPLY CURRENT (µA)
300
400
500
600
700
800
900
-60 -20-40 0 20 40 60 80 100
NO-LOAD SUPPLY CURRENT
vs. TEMPERATURE
MAX881R TOC07
TEMPERATURE (°C)
SUPPLY CURRENT (µA)
VIN = 5.5V
VIN = 3.6V
VIN = 2.5V
100
300
200
500
400
700
600
800
2.0 3.0 3.52.5 4.0 4.5 5.0 5.5 6.0
START-UP TIME
vs. INPUT VOLTAGE
MAX881R TOC08
INPUT VOLTAGE (V)
START-UP TIME (µs)
CIRCUIT OF FIGURE 3a
NO LOAD
SHDN TO POK DELAY
97
98
99
100
101
102
103
2.0 3.02.5 3.5 4.0 4.5 5.0 5.5 6.0
OSCILLATOR FREQUENCY
vs. INPUT VOLTAGE
MAX881R TOC09
INPUT VOLTAGE (V)
FREQUENCY (kHz)
TA = -40°C
TA = +25°C
TA = +85°C
MAX881R
Low-Noise Bias Supply in µMAX
with Power-OK for GaAsFET PA
4 _______________________________________________________________________________________
Typical Operating Characteristics (continued)
(Circuit of Figure 3, V
IN
= 3.6V, TA= +25°C, unless otherwise noted.)
100µs/div
START-UP FROM SHUTDOWN
(NO LOAD)
+5V
V
OUT
POK
SHDN
MAX881R-TOC10
-2V
0
0
V
IN
0
VIN = 3.6V
CIRCUIT OF FIGURE 3b
5µs/div
OUTPUT NOISE AND RIPPLE
(CIRCUIT OF FIGURE 3a)
V
OUT
1mV/div
AC COUPLED
MAX881R-TOC11
VIN = 3.6V
V
OUT
= -2V
I
OUT
= 2.5mA
10µs/div
OUTPUT NOISE AND RIPPLE
(CIRCUIT OF FIGURE 3b)
V
OUT
= 0
V
IN
= 0
1mV/div
AC COUPLED
V
OUT
= -2V
V
IN
= 3.6V
1mV/div
AC COUPLED
MAX881R-TOC12
RL = 500Ω
NOISE FLOOR
OUTPUT NOISE
60
-40
1 10 100 1000
OUTPUT NOISE SPECTRUM
(CIRCUIT OF FIGURE 3a)
-20
MAX881R TOC13
FREQUENCY (kHz)
NOISE (dBµV)
0
20
40
50
-30
-10
10
30
VIN = 3.6V
V
OUT
= -2V
I
OUT
= 2.5mA
60
-40
1 10 100 1000
OUTPUT NOISE SPECTRUM
(CIRCUIT OF FIGURE 3b)
-20
MAX881R TOC14
FREQUENCY (kHz)
NOISE (dBµV)
0
20
40
50
-30
-10
10
30
VIN = 3.6V
V
OUT
= -2V
I
OUT
= 4mA
2ms/div
LINE-TRANSIENT RESPONSE
V
IN
V
OUT
MAX881R-TOC15
10mV/
div
3.3V
2.7V
I
LOAD
= 4mA
V
OUT
= -2V
CIRCUIT OF FIGURE 3b
100µs/div
LOAD-TRANSIENT RESPONSE
I
OUT
V
OUT
MAX881R-TOC16
10mV/
div
4mA
400µA
V
IN
= 3.6V
V
OUT
= -2V
CIRCUIT OF FIGURE 3b
MAX881R
Low-Noise Bias Supply in µMAX
with Power-OK for GaAsFET PA
_______________________________________________________________________________________ 5
CHARGE PUMP
NEGOUT
SHDN
0.5V 0.4625V
GND
C1+
C1
C2
C4
C3
IN
OUT
FB
POK
C1-
MAX881R
Pin Description
GroundGND8
No Connection. Not internally connected.N.C.9
Positive Power-Supply InputIN10
Active-Low, Logic-Level Shutdown Input. Connect to IN for normal operation. Do not leave this pin
unconnected.
SHDN
5
Dual-Mode™ Feedback Input. When FB is connected to GND, the output is preset to -2V. To select
other voltages, connect FB to an external resistor-divider (Figure 4). Do not leave this pin
unconnected.
FB6
Regulated Negative Output VoltageOUT7
Active-Low, Open-Drain Power-OK Output. Goes low when OUT reaches 92.5% of its set value.
POK
4
Negative Output Voltage (unregulated)NEGOUT3
PIN
Negative Terminal for C1C1-2
Positive Terminal for C1C1+1
FUNCTIONNAME
Figure 1. Functional Diagram
Dual-Mode is a trademark of Maxim Integrated Products.
MAX881R
Low-Noise Bias Supply in µMAX
with Power-OK for GaAsFET PA
6 _______________________________________________________________________________________
________________Detailed Description
The MAX881R, a low-noise, inverting, regulated chargepump power supply, is designed for biasing GaAsFET
devices such as power-amplifier modules in cellular
handsets.
The applied input voltage (VIN) is inverted to a negative
voltage at NEGOUT by a capacitive charge pump. This
voltage is regulated by an internal linear regulator at
OUT (Figure 1). With FB connected to GND, V
OUT
is
regulated at -2V. Alternatively, use a voltage-divider at
FB to adjust the output voltage between -0.5V and -(V
IN
- 0.6V) (see the section
Setting the Output Voltage
).
The internal linear regulator reduces the ripple noise
induced by the charge-pump inverter to 1mVp-p at
OUT (circuit of Figure 3b). In addition, the excellent AC
rejection of the linear regulator attenuates noise from
the incoming supply.
Power-OK Signal
The MAX881R has an active-low, open-drain, power-OK
(POK) output. This output goes low when OUT reaches
92.5% of the regulated output voltage. POK can be
used to drive the gate of a P-MOSFET that switches
power to the GaAsFET power amplifier (Figure 2), thereby ensuring that the power amplifier is not powered
before the required negative bias voltage is present.
Use a 50kΩ or larger pull-up resistor to signal a logic
high when POK goes high impedance.
Shutdown Mode
The MAX881R features a shutdown mode that reduces
supply current to less than 1µA over temperature. SHDN
is an active-low, logic-level input. Start-up time coming
out of shutdown mode is typically 0.5ms. OUT and
NEGOUT are switched to GND in shutdown mode.
Applications Information
Setting the Output Voltage
Select either a fixed or adjustable output for the
MAX881R. Connect FB to GND for a fixed -2V output
(Figure 3). Select an alternative output voltage by connecting FB to the midpoint of a resistor-divider from
OUT to GND (Figure 4). When operating under full load
(4mA), the voltage at IN should be at least 0.6V higher
than the absolute voltage required at OUT. Note that
the minimum input voltage required for operation is
2.5V, regardless of the desired output voltage.
Choose R1 to be between 100kΩ and 400kΩ and calculate R2. For greater accuracy, use resistors with 1%
or better tolerance.
R2 = R1 (2 x |V
OUT
| - 1)
POK
100k
C4
V
CC
FOR
POWER AMPLIFIER
TO PA
GATE BIAS
TO
POWER
AMPLIFIER
OUT
MAX881R
P
Figure 2. Using the POK Function
NEGOUT
SHDN
C1-
C1+
POK
V
OUT
= -2V
C3
0.22µF
C2
0.22µF
C1
0.22µF
C4
4.7µF
ON
V
IN
OFF
OUT
IN
GND FB
MAX881R
Figure 3a. Standard Application Circuit for Minimum Capacitor
Values
NEGOUT
SHDN
C1-
C1+
POK
V
OUT
= -2V
C3
1µF
C2
1µF
C1
1µF
C4
10µF
V
IN
OUT
IN
GND FB
MAX881R
ON
OFF
Figure 3b. Standard Application Circuit for Minimum Output
Noise
MAX881R
Low-Noise Bias Supply in µMAX
with Power-OK for GaAsFET PA
_______________________________________________________________________________________ 7
Capacitors
Use capacitors with a low effective series resistance
(ESR) to maintain a low dropout voltage (VIN- |V
OUT
|).
The overall dropout voltage is a function of the output
resistance of the charge pump and the voltage drop
across the linear regulator (N-channel pass transistor).
At the 100kHz charge-pump switching frequency, output resistance is a function of the value of C1 and the
ESR of C1 and C2. Therefore, increasing C1 and minimizing the ESR of the charge-pump capacitors minimizes dropout voltage.
The output resistance of the entire circuit (in dropout) is
approximately:
R
OUT
= RO+ 4 x ESRC1+ ESRC2+ 1 / (fSx C1)
+ R
(linear regulator)
Where R
(linear regulator)
(the output impedance of the
linear regulator) is approximately 2Ω and RO(the resis-
tance of the internal switches) is typically 10Ω. When
regulating, the output resistance of the circuit is simply
the linear-regulator load regulation (2mV/mA).
C1, C2, and C3 should be 0.22µF capacitors with less
than 0.4Ω ESR. C4 should be a 4.7µF capacitor with
less than 0.1Ω ESR. Larger capacitor values can be
used (C1 = C2 = C3 = 1µF, C4 = 10µF) to reduce output noise and ripple (1mVp-p), at the expense of cost
and board space. All capacitors should be either
ceramic or surface-mount chip tantalum (Figures 3a
and 3b).
Layout and Grounding
Good layout is important for good noise performance.
To optimize the layout:
1) Mount all components as close together as possible.
2) Keep traces short to minimize parasitic inductance
and capacitance, especially 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 circuit and the oscilloscope
(which result from the charge pump’s switching action)
cause ground currents in the probes’ wires, inducing
sharp voltage spikes. For best results, measure directly
across the output capacitor (C4). Do not use the
ground lead of the probe; instead, remove the probe’s
tip cover and touch the ground ring on the probe
directly to C4’s ground terminal. This direct connection
gives the most accurate noise and ripple measurement.
NEGOUT
SHDN
C1-
C1+
POK
FB
C3
0.22µF
C2
0.22µF
C1
0.22µF
C4
4.7µF
V
IN
OUT
R2
R1
IN
GND
MAX881R
ON
OFF
Figure 4. Adjustable Output Configuration
TRANSISTOR COUNT: 413
Chip Information
MAX881R
Low-Noise Bias Supply in µMAX
with Power-OK for GaAsFET PA
8 _______________________________________________________________________________________
10LUMAXB.EPS
Package Information
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