LTC1261
10
1261fb
For more information www.linear.com/LTC1261
pump to respond until the next clock edge. This prevents
the charge 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
ESR
• (I
PK
/I
AVE
)Ω was placed in series
with the output. To avoid this nasty sequence of events
connect a 0.1µF ceramic capacitor in parallel with the
larger output capacitor. 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 capacitor supplies output current to the
load between charge cycles.
APPLICATIONS INFORMATION
LOW ESR
OUTPUT CAP
CLOCK
V
OUT
AVERAGE
V
SET
COMP1
OUTPUT
V
OUT
HIGH ESR
OUTPUT CAP
V
OUT
AVERAGE
V
SET
COMP1
OUTPUT
V
OUT
LTC1261 • F05
Figure 5. Output Ripple with Low and High ESR Capacitors
Figure 6. External Resistor Connections
Note that ESR in the flying capacitors will not cause the
same condition; in fact, it may actually improve the situ-
ation 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 capaci-
tors have to share charge at full duty cycle) the output
current capability of the LTC1261 will begin to diminish.
For 0.1µF flying capacitors, this gives a maximum total
series resistance of:
1
2
t
CLK
C
FLY
=
1
2
1
550kHz
/ 0.1µF = 9.1Ω
Most of this resistance is already provided by the internal
switches in the LTC1261 (especially in tripler mode). More
than 1Ω or 2Ω 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
OUT
(Figure 6). The LTC1261 uses a positive
reference with respect to V
OUT
, not a negative reference
with respect to ground (Figure 2 shows the reference con-
nection). 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.
LTC1261
GND
R1
6 (4*)
10 (5*)
11 (6*)
*LTC1261CS8
LTC1261 • F06
V
OUT
= –1.24V
R2
R1 + R2
R2
ADJ
OUT
( )
The 14-lead 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
ADJ
pins (Table 2). The internal resistors are roughly
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.
