SUPER CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
TC7660S
Detailed Description
The TC7660S contains all the necessary circuitry to
implement a voltage inverter, with the exception of two
external capacitors, which may be inexpensive 10
µF
polar-
ized electrolytic capacitors. Operation is best understood by
considering Figure 2, which shows an idealized voltage
inverter. Capacitor 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
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 four switches in 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 approach 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 will result in high power losses and probable
device latch-up.
This problem is eliminated in the TC7660S 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.
V+
S1
S2
1
2
C2
VOUT = – VIN
C1
GND
S3
S4
3
4
5
6
7
Figure 2. Idealized Charge Pump Inverter
The voltage regulator portion of the TC7660S is an
integral part of the anti-latch-up circuitry. Its inherent voltage
drop can, however, degrade operation at low voltages. To
improve low-voltage operation, the “LV” pin should be
connected to GND, disabling the regulator. For supply
voltages greater than 3.5V, the LV terminal must be left
open to ensure latch-up-proof operation and prevent device
damage.
Theoretical Power Efficiency
Considerations
In theory, a capacitive charge pump can approach
100% efficiency if certain conditions are met:
(1) The drive circuitry consumes minimal power.
(2) The output switches have extremely low ON
resistance and virtually no offset.
V+
1
2
C1
10µF
+
3
4
8
7
IS
V+
(+5V)
(3) The impedances of the pump and reservoir
capacitors are negligible at the pump frequency.
The TC7660S approaches these conditions for nega-
tive voltage multiplication 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
)
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.
4-71
TC7660S
6
5
COSC
*
IL
RL
VO
C2
10µF
+
NOTE:
For large values of C
OSC
(>1000pF), the values
of C
1
and C
2
should be increased to 100µF.
Figure 1. TC7660S Test Circuit
8
TELCOM SEMICONDUCTOR, INC.
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
