LTC1261CS datasheet(10/16 Pages) LINER

10 / 16 page

LTC1261

Most of this resistance is already provided by the internal

switches in the LTC1261 (especially in tripler mode). More

than 1

Ω 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 VOUT (Figure 6). The LTC1261 uses a positive

reference with respect to VOUT, 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

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

CESR × (IPK/IAVE)Ω was placed in series with the output. To

avoid this nasty sequence of events connect a 0.1

µF

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

VOUT

AVERAGE

VSET

COMP1

OUTPUT

VOUT

HIGH ESR

OUTPUT CAP

VOUT

AVERAGE

VSET

COMP1

OUTPUT

VOUT

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 0.1

µF

flying capacitors, this gives a maximum total series resis-

tance of:

/ 0.1

µF = 9.1Ω

)

) tCLK

CFLY

)

) 1

550kHz

Figure 5. Output Ripple with Low and High ESR Capacitors

Figure 6. External Resistor Connections

LTC1261

GND

6 (4*)

10 (5*)

11 (6*)

*LTC1261CS8

LTC1261 • F06

VOUT = –1.24V

R1 + 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

RADJ 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.

LTC1261CS Datasheet(PDF) 10 Page - Linear Technology

LTC1261CS Datasheet(HTML) 10 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Datasheet Download

Link URL