CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
TC7662B
DETAILED DESCRIPTION
The TC7662B contains all the necessary circuitry to
complete a negative voltage converter, with the exception of
two external capacitors which may be inexpensive 1µF
polarized electrolytic types. The mode of operation of the
device may be best understood by considering Figure 2,
which shows an idealized negative voltage converter. Ca-
pacitor C
1
is charged to a voltage V
+
for the half cycle when
switches S
1
and S
3
are closed. (Note: Switches S
2
and S
4
are open during this half cycle.) During the second half cycle
of operation, switches S
2
and S
4
are closed, with S
1
and S
3
open, thereby shifting capacitor C
1
negatively by V
+
volts.
Charge is then transferred from C
1
to C
2
such that the
voltage on C
2
is exactly V
+
, assuming ideal switches and no
load on C
2
. The TC7662B approaches this ideal situation
more closely than existing non-mechanical circuits.
In the TC7662B, the four switches of Figure 2 are MOS
power switches; S
1
is a P-channel device and S
2
, S
3
and S
4
are N-channel devices. The main difficulty with this ap-
proach is that in integrating the switches, the substrates of
S
3
and S
4
must always remain reverse biased with respect
to their sources, but not so much as to degrade their “ON”
resistances. In addition, at circuit start up, and under output
short circuit conditions (V
OUT
= V
+
), the output voltage must
be sensed and the substrate bias adjusted accordingly.
Failure to accomplish this would result in high power losses
and probable device latchup.
The problem is eliminated in the TC7662B by a logic
network which senses the output voltage (V
OUT
) together
with the level translators, and switches the substrates of S
3
and S
4
to the correct level to maintain necessary reverse
bias.
The voltage regulator portion of the TC7662B is an
integral part of the anti-latchup circuitry; however, its inher-
ent voltage drop can degrade operation at low voltages.
Therefore, to improve low voltage operation, the “LV” pin
should be connected to GND, disabling the regulator. For
supply voltages greater than 3.5 volts, the LV terminal must
be left open to insure latchup proof operation and prevent
device damage.
V+
1
2
C1 +
10
µF
3 TC7662B
4
8
7
6
5
RL
VO
C2
10
µF
IL
IS
V+
(+5V)
THEORETICAL POWER EFFICIENCY
CONSIDERATIONS
In theory, a voltage converter can approach 100%
efficiency if certain conditions are met:
A. The drive circuitry consumes minimal power.
B. The output switches have extremely low ON resistance
and virtually no offset.
C. The impedances of the pump and reservoir capacitors
are negligible at the pump frequency.
The TC7662B approaches these conditions for nega-
tive voltage conversion if large values of C
1
and C
2
are used.
Energy is lost only in the transfer of charge between
capacitors if a change in voltage occurs.
The energy lost
is defined by:
E = 1/2 C
1
(V
12
– V
22
)
where V
1
and V
2
are the voltages on C
1
during the pump and
transfer cycles. If the impedances of C
1
and C
2
are relatively
high at the pump frequency (refer to Figure 2) compared to
the value of R
L
, there will be a substantial difference in
voltages V
1
and V
2
. Therefore, it is desirable not only to
make C
2
as large as possible to eliminate output voltage
ripple, but also to employ a correspondingly large value for
C
1
in order to achieve maximum efficiency of operation.
Dos and Don’ts
1. Do not exceed maximum supply voltages.
2. Do not connect the LV terminal to GND for supply
voltages greater than 3.5 volts.
3. Do not short circuit the output to V
+
supply for voltages
above 5.5 volts for extended periods; however,
transient conditions including start-up are okay.
S1
VIN
C1
S2
S3
S4
C2
VOUT = – VIN
NOTE:
For large values of C
OSC
(>1000 pF), the values
of C
1
and C
2
should be increased to 100 µF.
+
Figure 1. TC7662B Test Circuit
© 2001 Microchip Technology Inc.
DS21469A
Figure 2. Idealized Negative Voltage Capacitor
3
TC7662B-8 9/11/96
MICROCHIP [ MICROCHIP TECHNOLOGY ]
