LTC1261(RevA) Ver la hoja de datos (PDF) - Linear Technology
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LTC1261 Datasheet PDF : 16 Pages
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LTC1261
APPLICATIONS INFORMATION
MODES OF OPERATION
The LTC1261 uses a charge pump to generate a negative
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-pin package.
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 voltage,
creating a voltage between VCC and OUT of up to two times
VS. 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 VCC and OUT pins
can be up to three times VS. For example, tripler mode can
be used to generate – 5V from a single positive 3.3V
supply. Tripler mode requires two external flying capaci-
tors. 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 dam-
aged. 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 8.5V supply. To avoid this problem use
doubler mode whenever possible with high input supply
voltages.
1
14
1
14
2 C1+
13
2 C1+
13
3 C1–
12 C1
3 C1–
12
1
8 C1
4 C2+ LTC1261 11
4 C2+ LTC1261 11
2 C1+
7
5 C2–
10 C2
5 C2–
10
C1
3 C1– LTC1261 6
6
9
6
9
4
5
7
8
7
8
a.) LTC1261CS8
DOUBLER MODE
b.) LTC1261CS
DOUBLER MODE
c.) LTC1261CS
TRIPLER MODE
LTC1261 • F01
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 VCC 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 volt-
age. 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 VCC
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 COUT,
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.
6
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