LTC1261CS8#TRPBF Linear Technology, LTC1261CS8#TRPBF Datasheet - Page 10

IC VOLT INVERT SW-CAP ADJ 8-SOIC

LTC1261CS8#TRPBF

Manufacturer Part Number
LTC1261CS8#TRPBF
Description
IC VOLT INVERT SW-CAP ADJ 8-SOIC
Manufacturer
Linear Technology
Type
Switched Capacitor (Charge Pump), Invertingr
Datasheet

Specifications of LTC1261CS8#TRPBF

Internal Switch(s)
Yes
Synchronous Rectifier
No
Number Of Outputs
1
Voltage - Output
-1.25 ~ -8 V
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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APPLICATIONS
LTC1261
pump from going into very high frequency oscillation
under such conditions but it also creates an output error
as the feedback loop regulates based on the top of the
spike, not the average value of the output (Figure 5). The
resulting output voltage behaves as if a resistor of value
C
avoid this nasty sequence of events connect a 0.1 F
ceramic capacitor in parallel with the larger output capaci-
tor. The ceramic capacitor will “eat” the high frequency
spike, preventing it from fooling the feedback loop, while
the larger but slower tantalum or aluminum output capaci-
tor supplies output current to the load between charge
cycles.
Note that ESR in the flying capacitors will not cause the
same condition; in fact, it may actually improve the situa-
tion by cutting the peak current and lowering the ampli-
tude of the spike. However, more flying capacitor ESR is
not necessarily better. As soon as the RC time constant
approaches half of a clock period (the time the capacitors
have to share charge at full duty cycle) the output current
capability of the LTC1261 will begin to diminish. For 0.1 F
flying capacitors, this gives a maximum total series resis-
tance of:
10
OUTPUT CAP
OUTPUT CAP
ESR
Figure 5. Output Ripple with Low and High ESR Capacitors
1
2
HIGH ESR
LOW ESR
CLOCK
C
t
(I
CLK
FLY
PK
/I
AVE
=
1
2
) was placed in series with the output. To
550kHz
U
V
V
OUT
OUT
1
INFORMATION
U
/ 0.1 F = 9.1
W
U
LTC1261 • F05
V
V
AVERAGE
COMP1
OUTPUT
V
V
AVERAGE
COMP1
OUTPUT
SET
OUT
SET
OUT
Most of this resistance is already provided by the internal
switches in the LTC1261 (especially in tripler mode). More
than 1 or 2 of ESR on the flying capacitors will start to
affect the regulation at maximum load.
RESISTOR SELECTION
Resistor selection is easy with the fixed output versions of
the LTC1261— no resistors are needed! Selecting the
right resistors for the adjustable parts is only a little more
difficult. A resistor divider should be used to divide the
signal at the output to give 1.24V at the ADJ pin with
respect to V
reference with respect to V
with respect to ground (Figure 2 shows the reference
connection). Be sure to keep this in mind when connecting
the resistors! If the initial output is not what you expected,
try swapping the two resistors.
The 14-pin adjustable parts include a built-in resistor
string which can provide an assortment of output voltages
by using different pin-strapping options at the R0, R1, and
R
124k, 226k, 100k, and 50k (see Figure 2) giving output
options of – 3.5V, – 4V, – 4.5V, and – 5V. The resistors are
carefully matched to provide accurate divider ratios, but
the absolute values can vary substantially from part to
part. It is not a good idea to create a divider using an
external resistor and one of the internal resistors unless
the output voltage accuracy is not critical.
ADJ
pins (Table 2). The internal resistors are roughly
Figure 6. External Resistor Connections
OUT
LTC1261
(Figure 6). The LTC1261 uses a positive
GND
OUT
ADJ
*LTC1261CS8
6 (4*)
10 (5*)
11 (6*)
OUT
R1
R2
, not a negative reference
V
OUT
= –1.24V
R1 + R2
LTC1261 • F06
R2

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