TC1044S-12 9/16/96
TC1044S
Charge Pump DC-TO-DC Voltage Converter
6
© 2001 Microchip Technology Inc. DS21348A
Figure 8. Positive Voltage Multiplier
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.
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 TC1044S 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
Figure 7. Lowering Oscillator Frequency
1
2
3
4
8
7
6
5
+
V
+
V
OUT
C
1
C
OSC
+
C
2
TC1044S
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.
Voltage Splitting
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 be-
tween 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).
1
2
3
4
8
7
6
5
V
+
V
OUT
=
(2 V
+
) – (2 V
F
)
+
C
2
D
1
D
2
+
C
1
TC1044S
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
+
C
2
TC1044S
D
2
+
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 TC1044S provides an inexpensive –5V source.
(See AN16 and AN17 for TC7135 interface details and
software routines.)
