LTC1261CS8-4.5#PBF Linear Technology, LTC1261CS8-4.5#PBF Datasheet - Page 6

IC SW CAP/INV -4.5V 15MA 8SOIC

LTC1261CS8-4.5#PBF

Manufacturer Part Number
LTC1261CS8-4.5#PBF
Description
IC SW CAP/INV -4.5V 15MA 8SOIC
Manufacturer
Linear Technology
Type
Switched Capacitor (Charge Pump), Invertingr
Datasheet

Specifications of LTC1261CS8-4.5#PBF

Internal Switch(s)
Yes
Synchronous Rectifier
No
Number Of Outputs
1
Voltage - Output
-4.5V
Current - Output
15mA
Frequency - Switching
550kHz
Voltage - Input
3 ~ 8 V
Operating Temperature
0°C ~ 70°C
Mounting Type
Surface Mount
Package / Case
8-SOIC (3.9mm Width)
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
Power - Output
-

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LTC1261
APPLICATIONS
6
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 V
V
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
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
can be up to three times V
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
second between C2
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
S
. In doubler mode the LT1261 uses a single flying
+
and C2
U
+
and C2
INFORMATION
pins. C1
S
U
. For example, tripler mode can
CC
(Figure 1c). Because of the
and OUT of up to two times
and C2
W
+
and C1
CC
+
and OUT pins
should be left
U
and the
C1
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.
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
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 V
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
connects C1
by S6. The charge in C1 and C2 is transferred to C
setting it to a negative voltage. Doubler mode works the
same way except that the single flying capacitor (C1) is
connected between C1
anything useful in doubler mode. C1 is charged initially by
S1 and S7 and connected to the output by S2 and S6.
1
2
3
4
DOUBLER MODE
a.) LTC1261CS8
C1
C1
+
LTC1261
Figure 1. Flying Capacitor Connections
to C2
8
7
6
5
C1
+
and C2
+
1
2
3
4
5
6
7
and C2
DOUBLER MODE
b.) LTC1261CS
C1
C1
C2
C2
+
+
LTC1261
is connected to the output
. S3, S4 and S5 don’t do
14
13
12
11
10
9
8
C1
C2
+
to ground, S4
1
2
3
4
5
6
7
TRIPLER MODE
c.) LTC1261CS
C1
C1
C2
C2
+
+
LTC1261
CC
LTC1261 • F01
OUT
and
CC
14
13
12
11
10
9
8
,

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