LTC1261CS8#TRPBF Linear Technology, LTC1261CS8#TRPBF Datasheet - Page 9

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
CAPACITOR SELECTION
Capacitor Sizing
The performance of the LTC1261 can be affected by the
capacitors it is connected to. The LTC1261 requires by-
pass capacitors to ground for both the V
The input capacitor provides most of LTC1261’s supply
current while it is charging the flying capacitors. This
capacitor should be mounted as close to the package as
possible and its value should be at least five times larger
than the flying capacitor. Ceramic capacitors generally
provide adequate performance but avoid using a tantalum
capacitor as the input bypass unless there is at least a
0.1 F ceramic capacitor in parallel with it. The charge
pump capacitors are somewhat less critical since their
peak currents are limited by the switches inside the
LTC1261. Most applications should use 0.1 F as the
flying capacitor value. Conveniently, ceramic capacitors
are the most common type of 0.1 F capacitor and they
work well here. Usually the easiest solution is to use the
same capacitor type for both the input bypass and the
flying capacitors.
In applications where the maximum load current is well-
defined and output ripple is critical or input peak currents
need to be minimized, the flying capacitor values can be
tailored to the application. Reducing the value of the
flying capacitors reduces the amount of charge trans-
ferred with each clock cycle. This limits maximum output
current, but also cuts the size of the voltage step at the
0.1 F
1 F
Figure 4. Output Filter Cuts Ripple Below 3mV
2
3
C1
C1
LTC1261CS8-4
+
GND
V
5V
CC
4
COMP
U
OUT
6
5
INFORMATION
U
100pF
+
W
3.3 F
10
LTC1261 • F04
CC
+
and OUT pins.
3.3 F
U
V
OUT
= –4V
output with each clock cycle. The smaller capacitors
draw smaller pulses of current out of V
peak currents and reducing the demands on the input
supply. Table 1 shows recommended values of flying
capacitor vs maximum load capacity.
Table 1. Typical Max Load (mA) vs Flying Capacitor Value at
T
The output capacitor performs two functions: it provides
output current to the load during half of the charge pump
cycle and its value helps to set the output ripple voltage.
For applications that are insensitive to output ripple, the
output bypass capacitor can be as small as 1 F. To achieve
specified output ripple with 0.1 F flying capacitors, the
output capacitor should be at least 3.3 F. Larger output
capacitors will reduce output ripple further at the expense
of turn-on time.
Capacitor ESR
Output capacitor Equivalent Series Resistance (ESR) is
another factor to consider. Excessive ESR in the output
capacitor can fool the regulation loop into keeping the
output artificially low by prematurely terminating the charg-
ing cycle. As the charge pump switches to recharge the
output a brief surge of current flows from the flying
capacitors to the output capacitor. This current surge can
be as high as 100mA under full load conditions. A typical
3.3 F tantalum capacitor has 1 or 2 of ESR; 100mA
2 = 200mV. If the output is within 200mV of the set point
this additional 200mV surge will trip the feedback com-
parator and terminate the charging cycle. The pulse dissi-
pates quickly and the comparator returns to the correct
state, but the RS latch will not allow the charge pump to
respond until the next clock edge. This prevents the charge
A
CAPACITOR
VALUE ( F)
= 25 C, V
FLYING
0.1
0.047
0.033
0.022
0.01
OUT
= – 4V
V
CC
= 5V DOUBLER MODE V
MAX LOAD (mA)
22
16
8
4
1
CC
CC
= 3.3V TRIPLER MODE
MAX LOAD (mA)
as well, limiting
LTC1261
20
15
11
5
3
9

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