Datasheet

LTC1261
9
1261fb
For more information www.linear.com/LTC1261
APPLICATIONS INFORMATION
Figure 4. Output Filter Cuts Ripple Below 3mV
LTC1261CS8-4
V
CC
5V
C1
+
C1
–
4
6
5
2
3
OUT
0.1µF
100pF
3.3µF
10Ω
COMP
LTC1261 • F04
GND
V
OUT
= –4V
1µF
++
3.3µF
CAPACITOR SELECTION
Capacitor Sizing
The performance of the LTC1261 can be affected by the
capacitors it is connected to. The LTC1261 requires bypass
capacitors to ground for both the V
CC
and OUT pins. 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 capaci-
tor as the input bypass unless there is at
least a 0.1µF
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 0.1µF as the
flying capaci-
tor value. Conveniently, ceramic capacitors are the most
common type of 0.1µF 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 transferred with
each clock cycle. This limits maximum output current, but
also cuts the size of the voltage step at the output with
each clock cycle. The smaller capacitors draw smaller
pulses of current out of V
CC
as well, limiting peak cur-
rents 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
A
= 25°C, V
OUT
= –4V
FLYING
CAPACITOR
VALUE (µF)
MAX LOAD (mA)
V
CC
= 5V DOUBLER MODE
MAX LOAD (mA)
V
CC
= 3.3V TRIPLER MODE
0.1 22 20
0.047 16 15
0.033 8 11
0.022 4 5
0.01 1 3
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 1µF. To achieve
specified output ripple with 0.1µF flying capacitors, the
output capacitor should be at least 3.3µF. 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
charging 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
3.3µF tantalum capacitor has 1Ω
or 2Ω of ESR; 100mA
• 2Ω = 200mV. If the output is within 200mV of the set
point this additional 200mV surge will trip the feedback
comparator and terminate the charging cycle. The pulse
dissipates quickly and the comparator returns to the
correct state, but the RS latch will not allow the charge