tc7660s Microchip Technology Inc., tc7660s Datasheet - Page 3

• Manufacturer Part Number: tc7660s
• Description: Super Charge Pump Dc-to-dc Voltage Converter
• Manufacturer: Microchip Technology Inc.

SUPER CHARGE PUMP DC-TO-DC
VOLTAGE CONVERTER
Detailed Description
implement a voltage inverter, with the exception of two
external capacitors, which may be inexpensive 10 F polar-
ized electrolytic capacitors. Operation is best understood by
considering Figure 2, which shows an idealized voltage
inverter. Capacitor C
cycle when switches S
S
half cycle of operation, switches S
S
V
volts. Charge is then transferred from C
voltage on C
load on C
S
devices. The main difficulty with this approach is that in
integrating the switches, the substrates of S
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
and the substrate bias adjusted accordingly. Failure to
accomplish this will result in high power losses and probable
device latch-up.
network which senses the output voltage (V
with the level translators, and switches the substrates of S
and S
bias.
© 2001 Microchip Technology Inc.
10 F
2
1
+
1
and S
and S
volts. Charge is then transferred from C
C 1
is a P-channel device, and S
The TC7660S contains all the necessary circuitry to
The four switches in Figure 2 are MOS power switches;
This problem is eliminated in the TC7660S by a logic
V +
NOTE: For large values of C
4
+
to the correct level to maintain necessary reverse
4
3
are open during this half cycle.) During the second
2
open, thereby shifting capacitor C
.
2
1
2
3
4
of C
OUT
is exactly V
TC7660S
Figure 1. TC7660S Test Circuit
1
= V
and C
1
+
), the output voltage must be sensed
is charged to a voltage V
1
DS21467A
2
and S
should be increased to 100 F.
+
, assuming ideal switches and no
8
7
6
5
3
are closed. (Note: Switches
OSC
2
, S
2
(>1000pF), the values
3
and S
and S
1
to C
1
4
C OSC
negatively by V+
+
4
are closed, with
1
2
are N-channel
3
, such that the
OUT
negatively by
and S
C 2
10 F
+
*
for the half
) together
I L
R L
I S
4
(+5V)
V +
must
V O
3
3
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
100% efficiency if certain conditions are met:
tive voltage multiplication if large values of C
used. Energy is lost only in the transfer of charge
between capacitors if a change in voltage occurs. The
energy lost is defined by:
transfer cycles. If the impedances of C
high at the pump frequency (refer to Figure 2) compared to
the value of R
voltages V
make C
ripple, but also to employ a correspondingly large value for
C
1
in order to achieve maximum efficiency of operation.
The voltage regulator portion of the TC7660S is an
In theory, a capacitive charge pump can approach
(1) The drive circuitry consumes minimal power.
(2) The output switches have extremely low ON
(3) The impedances of the pump and reservoir
The TC7660S approaches these conditions for nega-
V
1
V +
and V
2
GND
resistance and virtually no offset.
capacitors are negligible at the pump frequency.
as large as possible to eliminate output voltage
1
Figure 2. Idealized Charge Pump Inverter
and V
2
are the voltages on C
L
S 1
S 3
, there will be a substantial difference in
E = 1/2 C
2
. Therefore, it is desirable not only to
1
S 2
S 4
C 1
(V
1
2
– V
C 2
1
1
2
during the pump and
2
and C
)
2
TC7660S
V OUT = – V IN
1
are relatively
TC7660S-14 9/16/96
and C
2
are