The MAX840/MAX843/MAX844 low-noise, inverting
charge-pump power supplies are ideal for biasing
GaAsFETs in cellular telephone transmitter amplifiers.
They operate with inputs down to 2.5V.
The MAX840 offers both a -2V preset output and a -0.5V
to -9.4V adjustable output. The MAX843/MAX844 use
an external positive control voltage to set the negative
output voltage. Input voltage range for all the devices is
2.5V to 10V, and output current is 4mA with VIN> 2.7V.
These circuits can operate with small capacitors, as low
as 0.22µF.
An internal linear regulator reduces the MAX840’s output voltage ripple to 1mVp-p. With a well-filtered control
voltage (V
), the MAX843/MAX844 also achieve less
CTRL
than 1mVp-p typical output ripple. Supply current is
750µA, and reduces to less than 1µA in shutdown
(MAX840/MAX843). The MAX844’s unregulated output
is active in shutdown, with the charge pump switching at
20kHz. It provides a low-power LCD supply.
________________________Applications
Cellular Phones
GaAsFET Power Amplifier Modules
Personal Communicators, PDAs
Wireless Data Loggers
Continuously Adjustable GaAsFET Bias
LCD-Bias Contrast Control
Regulated Negative Power Supplies
__________Typical Operating Circuit
VIN = 2.5V to 10.0V
(3 CELLS)
____________________________Features
♦ Fixed -2V or Adjustable -0.5V to -9.4V Output
at 4mA (MAX840)
♦ 2.5V to 10V Input Voltage Range
♦ Operate with Small Capacitors (as low as 0.22µF)
♦ 1mVp-p Output Voltage Ripple
♦ Charge-Pump Switching Frequency:
100kHz in Normal Operation
20kHz in Shutdown Mode (MAX844)
♦ 1µA Max Logic-Level Shutdown Over Temp.
(MAX840/MAX843)
♦ Small 8-Pin SO Package
______________Ordering Information
PART
MAX840C/D
MAX840ISA
MAX840ESA-40°C to +85°C
MAX843C/D
MAX843ISA-25°C to +85°C8 SO
MAX843ESA-40°C to +85°C8 SO
MAX844C/D
MAX844ISA-25°C to +85°C8 SO
MAX844ESA-40°C to +85°C8 SO
For free samples & the latest literature: http://www.maxim-ic.com, or phone 1-800-998-8800
Low-Noise, Regulated, -2V GaAsFET Bias
ABSOLUTE MAXIMUM RATINGS
Supply Voltage, V
V
NEGOUT
V
IN
V
OUT
V
SHDN
to GND...................................................-10.5V to 0.3V
to V
NEGOUT
to GND (Note 1).......................................V
to GND ............................................-0.3V to (VIN+ 0.3V)
Continuous Power Dissipation (T
to GND ..................................-0.3V to 10.5V
IN
.........................................................-0.3V to 21V
to 0.3V
= +70°C)
A
NEGOUT
SO (derate 5.88mW/°C above +70°C).........................471mW
Note 1:The output may be shorted to NEGOUT or GND if the package power dissipation is not exceeded. Typical short-circuit
current from 4V to GND is 40mA.
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.
ELECTRICAL CHARACTERISTICS
(Figures 2a and 2c, 2.5V ≤ VIN≤ 10V, V
Typical values are measured at VIN= 3.6V and TA= +25°C.)
Supply Voltage Range
Output Voltage
MAX840/MAX843/MAX844
Output Voltage Adjust Range
V
V
FB Voltage
FB Leakage Current
CONT Leakage Current
CONT
Supply CurrentµA7501300
Shutdown Supply Current
V
Load Regulation
OUT
Ripple
OUT
I
SHUT
Oscillator FrequencyF
Input High VoltageV
Input Low VoltageV
Input CurrentI
Input CapacitanceC
= -2V, GND = 0V, RL= ∞, SHDN = VIN, TA= T
OUT
IN
MAX840, VFB= 0V
OUT
MAX843/MAX844,
V
= 2V
CTRL
OUT
MAX840, no load
FB
FB
MAX840, VFB= -0.5V
MAX843/MAX844, V
No load, VIN≤ 3.6VI
1C1+Positive Terminal for C1
2C1-Negative Terminal for C1
3NEGOUTNegative Output Voltage (unregulated)
4SHDNActive-Low, TTL Logic-Level Shutdown Input
5FB
—CONT
6OUTRegulated Negative Output Voltage
7GNDGround
8INPositive Power-Supply Input
Dual Mode is a trademark of Maxim Integrated Products.
MAX843
MAX844
1
2
3
4
—
5
6
7
8
NAMEFUNCTION
Dual Mode™ Feedback Input. When FB is grounded, the output is preset to -2V. To
select other output voltages, connect FB to an external resistor divider (Figure 2b).
Control Voltage Input. To set V
, connect a resistor divider between OUT and a
OUT
positive control voltage between 0V and 10V (Figure 2c).
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 (VIN) is first inverted to a negative 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
lator’s excellent AC rejection attenuates noise from the
incoming supply.
. In addition, the linear regu-
OUT
__________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 voltage by connecting FB to the midpoint of a resistor
voltage divider from OUT to GND (Figure 2b). V
MAX840/MAX843/MAX844
must be 0.6V above the absolute value of V
allow proper regulation. The output voltage is calculated from the formula below. Choose R2 to be between
100kΩ and 400kΩ.
V
= (-0.5V)(1 + R2 / R1)
OUT
For the MAX843/MAX844, set the output voltage by
connecting a resistor voltage divider between OUT and
a positive control voltage (V
V
= -V
OUT
CTRL
) (Figure 2c).
CTRL
(R2 / R1)
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 disabled. When the MAX844 is in shutdown, only the linear regulator is disabled and the NEGOUT output
remains enabled. However, the charge-pump oscillation 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 shutdown.
OUT
IN
to
Capacitors
Use capacitors with low effective series resistance
(ESR) to maintain a low dropout voltage (VIN- |V
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 approximately:
R
= RO+ 4 x ESRC1+ ESRC4+
OUT
1 / (fSx C1) + R
where [RO+ R
of the internal switches and the resistance across the
linear regulator, is approximately 71Ω at VIN= 2.5V,
48Ω at VIN= 5V, and 40Ω at VIN= 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 tantalum or ceramic types. External capacitor values can be
adjusted to optimize size and cost.
(linear regulator)
(linear regulator)
], the effective resistance
OUT
|
Layout and Grounding
Good layout is important, primarily for good noise performance. 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.