Applications information – Rainbow Electronics MAX853 User Manual
Page 7
MAX850–MAX853
Low-Noise, Regulated, Negative
Charge-Pump Power Supplies for GaAsFET Bias
_______________________________________________________________________________________
7
__________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). V
IN
must
be 1.0V above the absolute value of V
OUT
to allow proper
regulation. The output voltage is calculated from the formu-
la below. Choose R2 to be between 100k
Ω
to 400k
Ω
.
For the MAX853, set the output voltage, VOUT, by con-
necting a resistor voltage divider between OUT and a
positive control voltage, V
CTRL
(Figure 2c.)
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 (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 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, 0.8
Ω
ESR capacitors are recommended for C1, C2,
and C3. C4 should be 10µF, 0.2
Ω
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
MAX850
MAX851
MAX852
FB
OUT
SHDN*
SHDN
OSC
NEGOUT
C1-
C1+
IN
GND
V
IN
C3
1
µ
F
V
OUT
= -4.1V
(V
GG
of GaAsFET)
*MAX850: SHDN
MAX851: SHDN
MAX852: OSC
C4
10
µ
F
C1
1
µ
F
C2
1
µ
F
MAX853
CONT
OUT
SHDN
NEGOUT
C1-
C1+
IN
GND
V
IN
C3
1
µ
F
V
OUT
= -0.5V to -9V @ 5mA
C4
10
µ
F
C1
1
µ
F
C2
1
µ
F
R2
100k
R1
100k
V
CTRL
(0V TO 10V)
Figure 2a. MAX850/MAX851/MAX852 Standard Application Circuit
Figure 2c. MAX853 Standard Application Circuit
MAX850
MAX851
MAX852
FB
OUT
SHDN*
SHDN
OSC
NEGOUT
C1-
C1+
IN
GND
V
IN
C3
1
µ
F
V
OUT
= (-1.28V) 1+
R2
R1
*MAX850: SHDN
MAX851: SHDN
MAX852: OSC
C4
10
µ
F
C1
1
µ
F
C2
1
µ
F
R2
100k
R1
100k
( )
Figure 2b. MAX850/MAX851/MAX852 Adjustable Configuration
V
= -1.28
OUT
(
)
+
1
2
1
R
R
V
= - V
R2
R1
OUT
CTRL