Rainbow Electronics MAX865 - Manual (Page 5)

MAX865
Compact, Dual-Output Charge Pump
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_______________Detailed Description
The MAX865 contains all the circuitry needed to imple-
ment a voltage doubler/inverter. Only four external
capacitors are needed. These may be polarized elec-
trolytic or ceramic capacitors with values ranging from
1µF to 100µF.
Figure 2a shows the ideal operation of the positive volt-
age doubler. The on-chip oscillator generates a 50%
duty-cycle clock signal. During the first half cycle,
switches S2 and S4 open, switches S1 and S3 close,
and capacitor C1 charges to the input voltage (V
IN ).
During the second half cycle, switches S1 and S3
open, switches S2 and S4 close, and capacitor C1 is
level shifted upward by V
IN . Assuming ideal switches
and no load on C3, charge transfers into C3 from C1
such that the voltage on C3 will be 2V
IN , generating the
positive supply output (V+).
Figure 2b illustrates the ideal operation of the negative
converter. The switches of the negative converter are
out of phase with the positive converter. During the
second half cycle, switches S6 and S8 open and
switches S5 and S7 close, charging C2 from V+
(pumped up to 2V
IN by the positive charge pump) to
GND. In the first half of the clock cycle, switches S5
and S7 open, switches S6 and S8 close, and the
charge on capacitor C2 transfers to C4, generating the
negative supply. The eight switches are CMOS power
MOSFETs. Switches S1, S2, S4, and S5 are P-channel
devices, while switches S3, S6, S7, and S8 are N-chan-
nel devices.
Charge-Pump Output
The MAX865 is not a voltage regulator: the output
source resistance of either charge pump is approxi-
mately 150
Ω
at room temperature with V IN = +5V, and
V+ and V- will approach +10V and -10V, respectively,
when lightly loaded. Both V+ and V- will droop toward
GND as the current draw from either V+ or V- increas-
es, since V- is derived from V+. Treating each convert-
er separately, the droop of the negative supply
(V
DROOP- ) is the product of the current draw from V-
(I V- ) and the source resistance of the negative convert-
er (RS-):
The droop of the positive supply (V DROOP+ ) is the
product of the current draw from the positive supply
(I
LOAD+ ) and the source resistance of the positive
IN
a) b)
S1
S3
C1+
C1 C3
C1-
S2
S4
S5 S6
S7 S8
C2-
GND
V-
R L -
R L +
C2+
C4
C2
GND I N
I V -
GND
I V +
V+ V+
Figure 2. Idealized Voltage Quadrupler: a) Positive Charge Pump; b) Negative Charge Pump
V = I x RS -
DROOP - V -