MC7660
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5
Figure 5. Increased Output Voltage by Cascading Devices
MC7660
1
2
3
4
8
7
6
5
V+
MC7660
1
2
3
4
8
7
6
5
+
10 F
+
10 F
10 F
+
VOUT*
*NOTE:
VOUT = –n V+ for 1.5 V
≤
V+
≤
10 V
“1”
“n”
Figure 6. External Clocking
MC7660
1
2
3
4
8
7
6
5
V+
+
+
10 F
VOUT
V+
1 k
CMOS
GATE
10 F
Figure 7. Lowering Oscillator Frequency
MC7660
1
2
3
4
8
7
6
5
V+
C1
+
+
C2
VOUT
COSC
Positive Voltage Multiplication
The MC7660 may be employed to achieve positive
voltage multiplication using the circuit shown in Figure 8.
In this application, the pump inverter switches of the
MC7660 are used to charge C
1
to a voltage level of V
+
– V
F
(where V
+
is the supply voltage and V
F
is the forward voltage
drop of diode D
1
). On the transfer cycle, the voltage on C
1
plus the supply voltage (V
+
) is applied through diode D
2
to
capacitor C
2
. The voltage thus created on C
2
becomes (2 V
+
)
– (2 V
F
), or twice the supply voltage minus the combined
forward voltage drops of diodes D
1
and D
2
.
The source impedance of the output (V
OUT
) will depend on
the output current, but for V
+
= 5V and an output current of
10 mA, it will be approximately 60 .
Figure 8. Positive Voltage Multiplier
MC7660
1
2
3
4
8
7
6
5
V+
+
C2
VOUT =
(2 V+) – (2 VF)
D1
C1
D2
+
Combined Negative Voltage Conversion and Positive
Supply Multiplication
Figure 9 combines the functions shown in Figures 3 and
8 to provide negative voltage conversion and positive
voltage multiplication simultaneously. This approach would
be, for example, suitable for generating +9V and –5V from
an existing +5V supply. In this instance, capacitors C
1
and
C
3
perform the pump and reservoir functions, respectively,
for the generation of the negative voltage, while capacitors
C
2
and C
4
are pump and reservoir, respectively, for the
multiplied positive voltage. There is a penalty in this
configuration which combines both functions, however, in
that the source impedances of the generated supplies will be
somewhat higher due to the finite impedance of the common
charge pump driver at pin 2 of the device.
Figure 9. Combined Negative Converter and
Positive Muliplier
MC7660
1
2
3
4
8
7
6
5
V+
+
C4
VOUT =
–(V+ – VF)
D1
D2
+
C3
VOUT =
(2 V+) – (2 VF)
+
C2
+
C1