MAX1044/ICL7660
Switched-Capacitor Voltage Converters
Pin Description
PIN
NAME
BOOST
(MAX1044)
N.C.
(ICL7660)
CAP+
GND
CAP-
V
OUT
LV
OSC
V+
FUNCTION
Frequency Boost. Connecting BOOST to V+ increases the oscillator frequency by a factor of six.
When the oscillator is driven externally, BOOST has no effect and should be left open.
No Connection
Connection to positive terminal of Charge-Pump Capacitor
Ground. For most applications, the positive terminal of the reservoir capacitor is connected to this pin.
Connection to negative terminal of Charge-Pump Capacitor
Negative Voltage Output. For most applications, the negative terminal of the reservoir capacitor is
connected to this pin.
Low-Voltage Operation. Connect to ground for supply voltages below 3.5V.
ICL7660: Leave open for supply voltages above 5V.
Oscillator Control Input. Connecting an external capacitor reduces the oscillator frequency. Minimize stray
capacitance at this pin.
Power-Supply Positive Voltage Input. (1.5V to 10V). V+ is also the substrate connection.
1
2
3
4
5
6
7
8
V+
BOOST
V+
C
BYPASS
= 0.1µF
EXTERNAL
OSCILLATOR
C
OSC
GND
LV
R
L
CAP+
C1
10µF
MAX1044
ICL7660
OSC
terminal of C1 to ground and shifts the negative terminal
to V
OUT
. This connects C1 in parallel with the reservoir
capacitor C2. If the voltage across C2 is smaller than the
voltage across C1, then charge flows from C1 to C2 until the
voltages across them are equal. During successive cycles,
C1 will continue pouring charge into C2 until the voltage
across C2 reaches - (V+). In an actual voltage inverter, the
output is less than - (V+) since the switches S1–S4 have
resistance and the load drains charge from C2.
Additional qualities of the MAX1044/ICL7660 can be
understood by using a switched-capacitor circuit model.
Switching the bucket capacitor, C1, between the input and
output of the circuit synthesizes a resistance (Figures 3a
and 3b.)
When the switch in Figure 3a is in the left position,
capacitor C1 charges to V+. When the switch moves
to the right position, C1 is discharged to V
OUT
. The
charge transferred per cycle is: ΔQ = C1(V+ - V
OUT
).
If the switch is cycled at frequency f, then the resulting
current is: I = f x ΔQ = f x C1(V+ - V
OUT
). Rewriting
this equation in Ohm’s law form defines an equivalent
resistance synthesized by the switched-capacitor circuit
where:
I
=
CAP-
V
OUT
C2
10µF
V
OUT
Figure 1. Maxim MAX1044/ICL7660 Test Circuit
Detailed Description
The MAX1044/ICL7660 are charge-pump voltage
converters. They work by first accumulating charge in
a bucket capacitor and then transfer it into a reservoir
capacitor. The ideal voltage inverter circuit in Figure 2
illustrates this operation.
During the first half of each cycle, switches S1 & S3
close and switches S2 & S4 open, which connects the
bucket capacitor C1 across V+ and charges C1. During the
second half of each cycle, switches S2 & S4 close and
switches S1 & S3 open, which connects the positive
(
V
+
−
V
OUT
)
1/ ( f x C1)
and
1
R
EQUIV
=
f x C1
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