LTC1261
6
1261fb
For more information www.linear.com/LTC1261
APPLICATIONS INFORMATION
MODES OF OPERATION
The LTC1261 uses a charge pump to generate a nega-
tive output voltage that can be regulated to a value either
higher or lower than the original input voltage. It has two
modes of operation: a “doubler” inverting mode, which
can provide a negative output equal to or less than the
positive power supply and a “tripler” inverting mode,
which can provide
negative output voltages either larger
or smaller in magnitude than the original positive supply.
The tripler offers greater versatility and wider input range
but requires four external capacitors and a 14-lead pack-
age. The doubler offers the SO-8 package and requires
only three external capacitors.
Doubler Mode
Doubler mode allows the LTC1261 to generate negative
output voltage magnitudes up to that of the supply volt-
age, creating
a voltage between V
CC
and OUT of up to two
times V
S
. In doubler mode the LT1261 uses a single flying
capacitor to invert the input supply voltage, and the output
voltage is stored on the output bypass capacitor between
switch cycles. The LTC1261CS8 is always configured in
doubler mode and has only one pair of flying capacitor
pins (Figure 1a). The LTC1261CS
can be configured in
doubler mode by connecting a single flying capacitor
between the C1
+
and C2
–
pins. C1
–
and C2
+
should be
left floating (Figure 1b).
Tripler Mode
The LTC1261CS can be used in a tripler mode which can
generate negative output voltages up to twice the supply
voltage. The total voltage between the V
CC
and OUT pins
can be up
to three times V
S
. For example, tripler mode
can be used to generate – 5V from a single positive 3.3V
supply. Tripler mode requires two external flying capacitors.
The first connects between C1
+
and C1
–
and the second
between C2
+
and C2
–
(Figure 1c). Because of the relatively
high voltages that can be generated in this mode, care must
be taken to ensure
that the total input-to-output voltage
never exceeds 12V or the LTC1261 may be damaged. In
most applications the output voltage will be kept in check
by the regulation loop. Damage is possible however, with
supply voltages above 4V in tripler mode and above 6V
in doubler mode. As the input supply voltage rises the
allowable output voltage drops, finally reaching –4V with
an 8.5
V supply. To avoid this problem use doubler mode
whenever possible with high input supply voltages.
1
2
3
4
5
6
7
14
13
12
11
10
9
8
C1
C2
C1
+
C1
–
C2
+
LTC1261
LTC1261 • F01
C2
–
1
2
3
4
5
6
7
14
13
12
11
10
9
8
C1
C1
+
C1
–
C2
+
LTC1261
C2
–
1
2
3
4
8
7
6
5
C1
+
C1
–
C1 LTC1261
a.) LTC1261CS8
DOUBLER MODE
b.) LTC1261CS
DOUBLER MODE
c.) LTC1261CS
TRIPLER MODE
Figure 1. Flying Capacitor Connections
THEORY OF OPERATION
A block diagram of the LTC1261 is shown in Figure 2. The
heart of the LTC1261 is the charge pump core shown in the
dashed box. It generates a negative output voltage by first
charging the flying capacitors between V
CC
and ground.
It then stacks the flying capacitors on top of each other
and connects the top of the stack to
ground forcing the
bottom of the stack to a negative voltage. The charge on
the flying capacitors is transferred to the output bypass
capacitor, leaving it charged to the negative output voltage.
This process is driven by the internal clock.
Figure 2 shows the charge pump configured in tripler mode.
With the clock low, C1 and C2 are charged to V
CC
by S1,
S3, S5 and S7.
At the next rising clock edge, S1, S3, S5
and S7 open and S2, S4 and S6 close, stacking C1 and
C2 on top of each other. S2 connects C1
+
to ground, S4
connects C1
–
to C2
+
and C2
–
is connected to the output
by S6. The charge in C1 and C2 is transferred to C
OUT
,
setting it to a negative voltage. Doubler
mode works the
same way except that the single flying capacitor (C1) is
connected between C1
+
and C2
–
. S3, S4 and S5 don’t do
anything useful in doubler mode. C1 is charged initially
by S1 and S7 and connected to the output by S2 and S6.
