MAX850 データシートの表示（PDF） - Maxim Integrated
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MAX850 Datasheet PDF : 8 Pages
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Low-Noise, Regulated, Negative
Charge-Pump Power Supplies for GaAsFET Bias
__________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-
tor voltage divider from OUT to GND (Figure 2b). VIN must
be 1.0V above the absolute value of VOUT to allow proper
regulation. The output voltage is calculated from the formu-
la below. Choose R2 to be between 100kΩ to 400kΩ.
( ) VOUT =
-1.28
1+
R2
R1
C1
1µF
C2
1µF
*MAX850: SHDN
MAX851: SHDN
MAX852: OSC
VIN
IN
C1+
MAX850
C1- MAX851 OUT
MAX852
NEGOUT
SHDN*
SHDN
FB
OSC
GND
C3
1µF
VOUT = -4.1V
(VGG of GaAsFET)
C4
10µF
Figure 2a. MAX850/MAX851/MAX852 Standard Application Circuit
VIN
C3
IN
1µF
C1+
C1
1µF
( ) C1-
MAX850
MAX851
OUT
VOUT = (-1.28V)
1+ R2
R1
MAX852
C4
10µF
C2
NEGOUT
R2
1µF
100k
SHDN*
SHDN
FB
OSC
GND
R1
100k
*MAX850: SHDN
MAX851: SHDN
MAX852: OSC
Figure 2b. MAX850/MAX851/MAX852 Adjustable Configuration
VIN
C3
IN
1µF
C1+
C1
1µF
VOUT = -0.5V to -9V @ 5mA
C1- MAX853 OUT
C4
C2
NEGOUT
1µF
R2
10µF
100k
SHDN
CONT
GND
R1
100k
VCTRL (0V TO 10V)
Figure 2c. MAX853 Standard Application Circuit
For the MAX853, set the output voltage, VOUT, by con-
necting a resistor voltage divider between OUT and a
positive control voltage, VCTRL (Figure 2c.)
VOUT
=
-
VCTRL
R2
R1
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
maintain a low dropout voltage (VIN - |VOUT|). 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 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, 0.8Ω ESR capacitors are recommended for C1, C2,
and C3. C4 should be 10µF, 0.2Ω 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
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