Datasheet

6
TC7662B
CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
TC7662B-8 9/11/96
© 2001 Microchip Technology Inc. DS21469A
1
2
3
4
8
7
6
5
+
V
+
V
OUT
C
1
C
OSC
+
C
2
TC7662B
Figure 8. Lowering Oscillator Frequency
Positive Voltage Doubling
The TC7662B may be employed to achieve positive
voltage doubling using the circuit shown in Figure 9. In this
application, the pump inverter switches of the TC7662B 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 on C
1
plus
the supply voltage (V
+
) 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 60Ω.
Combined Negative Voltage Conversion
and Positive Supply Multiplication
Figure 10 combines the functions shown in Figures 3
and 9 to provide negative voltage conversion and positive
voltage doubling 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 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.
1
2
3
4
8
7
6
5
+
V
+
V
OUT
=
(2 V
+
) – (2 V
F
)
C
1
D
1
+
+
C
3
C
4
V
OUT
=
–(V
+
–V
F
)
C
2
TC7662B
D
2
+
Figure 10. Combined Negative Converter and Positive Doubler
Figure 9. Positive Voltage Multiplier
Voltage Splitting
The bidirectional characteristics can also be used to
split a higher supply in half, as shown in Figure 11. The
combined load will be evenly shared between the two sides
and a high value resistor to the LV pin ensures start-up.
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 6, +15V can be
converted (via +7.5V and –7.5V) to a nominal –15V, though
with rather high series resistance (~250Ω).
1
2
3
4
8
7
6
5
V
+
V
OUT
=
(2 V
+
) – (2 V
F
)
+
C
2
D
1
D
2
+
C
1
TC7662B
+
R
L1
R
L2
V
OUT
=
V
+
–V
–
2
50
µF
50
µF
V
+
V
–
50 µF
+
1
2
8
7
TC7662B
3
4
6
5
-
-
+
-
Figure 11. Splitting a Supply in Half