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TL7660
+
-
+
-
+
-
+
-
V+
C
OSC
V
OUT
C
1
C
2
Positive Voltage Doubling
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2
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4
8
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5
TL7660
V
+
V = (2V+) – (2VF)
OUT
D
1
D
2
C
1
C
2
TL7660
CMOS VOLTAGE CONVERTER
SCAS794 – JUNE 2006
APPLICATION INFORMATION (continued)
It is also possible to increase the conversion efficiency of the TL7660 at low load levels by lowering the oscillator
frequency (see Figure 8 ). This reduces the switching losses. However, lowering the oscillator frequency causes
an undesirable increase in the impedance of the pump (C
1
) and reservoir (C
2
) capacitors; this is overcome by
increasing the values of C
1
and C
2
by the same factor that the frequency has been reduced. For example, the
addition of a 100-pF capacitor between terminal 7 (OSC) and V
CC
lowers the oscillator frequency to 1 kHz from
its nominal frequency of 10 kHz (a multiple of 10), and thereby necessitate a corresponding increase in the
value of C
1
and C
2
(from 10 µ F to 100 µ F).
Figure 8. Lowering Oscillator Frequency
The TL7660 may be used to achieve positive voltage doubling using the circuit shown in Figure 9 . In this
application, the pump inverter switches of the TL7660 are used to charge C
1
to a voltage level of V
CC
– V
F
(where V
CC
is the supply voltage and V
F
is the forward voltage drop of diode D1). On the transfer cycle, the
voltage on C
1
plus the supply voltage (V
CC
) is applied through diode D
2
to capacitor C
2
. The voltage thus
created on C
2
becomes (2V
CC
) – (2V
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
) depends on the output current.
Figure 9. Positive-Voltage Doubler
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