TC7662A
4.5
Changing the TC7662A Oscillator
Frequency
4.7
Combined Negative Voltage
Conversion and Positive Supply
Multiplication
It
is possible to increase the conversion efficiency of
the TC7662A at low load levels by lowering the
oscillator frequency. This reduces the switching losses,
and is shown in Figure 4-5. However, lowering the
oscillator frequency will cause an undesirable increase
in the impedance of the pump (C
P
) and reservoir (C
R
)
capacitors; this is overcome by increasing the values of
C
P
and C
R
by the same factor that the frequency has
been reduced. For example, the addition of a 100pF
capacitor between pin 7 (OSC) and V
DD
will lower the
oscillator frequency to 2kHz from its nominal frequency
of 12kHz (multiple of 6), and thereby necessitate a
corresponding increase in the value of C
P
and C
R
(from
10µF to 68µF).
Figure 4-7 combines the functions shown in Figure 4-1
and Figure 4-6 to provide negative voltage conversion
and positive voltage doubling simultaneously. This
approach would be, for example, suitable for generat-
ing +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 doubled
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 4-5:
LOWERING OSCILLATOR
FREQUENCY
V
DD
FIGURE 4-7:
COMBINED NEGATIVE
CONVERTER AND
POSITIVE DOUBLER
V
DD
V
OUT
=
-(V
DD
– V
F
)
1
2
10µF
+
3
4
8
7
1
8
7
TC7662A
6
5
C
OSC
V
OUT
10µF
+
+
C
1
2
TC7662A
3
4
+
C
2
6
5
D
1
+
C
3
D
2
V
OUT
=
(2 V
DD
) – (2 V
F
)
+
C
4
4.6
Positive Voltage Doubling
4.8
Voltage Splitting
The TC7662A may be employed to achieve positive
voltage doubling using the circuit shown in Figure 4-6.
In this application, the pump inverter switches of the
TC7662A are used to charge C
P
to a voltage level of
V
DD
– V
F
(where V
DD
is the supply voltage and V
F
is
the forward voltage on C
P
plus the supply voltage (V
DD
)
applied through diode D
2
to capacitor C
R
). The voltage
thus created on C
R
becomes (2 V
DD
) – (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
DD
= 5V and an output
current of 10 mA, it will be approximately 60Ω.
The same bidirectional characteristics can be used to
split a higher supply in half, as shown in Figure 4-8.
The combined load will be evenly shared between the
two sides. Because the switches share the load in
parallel, the output impedance is much lower than in
the standard circuits, and higher currents can be drawn
from the device. By using this circuit, and then the
circuit of Figure 4-4, +15V can be converted (via +7.5V
and -7.5V) to a nominal -15V, though with rather high
series resistance (~250Ω).
FIGURE 4-8:
FIGURE 4-6:
POSITIVE VOLTAGE
MULTIPLIER
V
DD
+
R
L1
V
OUT
=
V
DD
– V – 50
µF
2
R
L2
+
–
50µF
–
SPLITTING A SUPPLY IN
HALF
V
DD
1
2
8
7
1
2
3
4
8
7
D
1
D
2
+
C
P
+
C
R
V
OUT
=
(2 V
DD
) – (2 V
F
)
+
–
TC7662A
3
4
6
5
TC7662A
6
5
50µF
V–
DS21468B-page 8
2002 Microchip Technology Inc.
MICROCHIP [ MICROCHIP TECHNOLOGY ]
