SUPER CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
TC7660S
V
1
2
C1
+
3
4
8
7
+
COSC
will bypass the other (D
1
and D
2
in Figure 9 would never turn
on), or else the diode and resistor shown dotted in Figure 10
can be used to "force" the internal regulator on.
TC7660S
Voltage Splitting
VOUT
+
C2
6
5
Figure 7. Lowering Oscillator Frequency
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 volt-
age 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 multi-
plied 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.
The same bidirectional characteristics used in Figure 10
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. Once again, 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 5,
+15V can be converted (via +7.5V and – 7.5V) to a nominal
–15V, though with rather high series resistance (~250Ω).
V+
VOUT = –V+
1
2
3
+
C1
4
+
C2
8
7
+
C3
TC7660S
6
5
D1
D2
VOUT =
(2 V +) – (2 VF)
+
C4
Efficient Positive Voltage
Multiplication/Conversion
Since the switches that allow the charge pumping op-
eration are bidirectional, the charge transfer can be per-
formed backwards as easily as forwards. Figure 10 shows
a TC7660S transforming – 5V to +5V (or +5V to +10V, etc.).
The only problem here is that the internal clock and switch-
drive section will not operate until some positive voltage has
been generated. An initial inefficient pump, as shown in
Figure 9, could be used to start this circuit up, after which it
V+
1
2
3
4
8
7
D1
D2
+
C1
+
C2
VOUT =
(2 V+) – (2 VF)
Figure 9. Combined Negative Converter and Positive Multiplier
Negative Voltage Generation for
Display ADCs
The TC7106 is designed to work from a 9V battery. With
a fixed power supply system, the TC7106 will perform
conversions with input signal referenced to power supply
ground.
Negative Supply Generation for
4¹⁄₂ Digit Data Acquisition System
The TC7135 is a 4¹⁄₂ digit ADC operating from
±5V
supplies. The TC7660S provides an inexpensive –5V source.
(See AN16 and AN17 for TC7135 interface details and
software routines.)
TC7660S
6
5
Figure 8. Positive Voltage Multiplier
TC7660S-14 9/16/96
6
© 2001 Microchip Technology Inc.
DS21467A
MICROCHIP [ MICROCHIP TECHNOLOGY ]
