Datasheet

10
LTC1261
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
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.
APPLICATIONS INFORMATION
WUU
U
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 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.
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
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 0.1µF
flying capacitors, this gives a maximum total series resis-
tance of:
= / 0.1µF = 9.1Ω
1
2
) )
t
CLK

C
FLY
1
2
) )
1
550kHz
Figure 5. Output Ripple with Low and High ESR Capacitors
Figure 6. External Resistor Connections
LTC1261
GND
R1
6 (4*)
10 (5*)
11 (6*)
*LTC1261CS8
LTC1261 • F06
V
OUT
= –1.24V
R2
R1 + R2
R2
ADJ
OUT
()
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
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.