10

LTC1429

going into very high frequency oscillation under such condi-

tions. 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 7). The resulting output voltage behaves

with the output. To minimize this effect, output capacitor ESR

should be as low as possible or smaller value high frequency

bypass (typically a 0.1

µFceramic)shouldbeaddedinparallel

with the output capacitor.

Figure 7. Output Ripple with Low and High ESR Caps

LOW ESR

OUTPUT CAP

CLOCK

AVERAGE

COMP1

OUTPUT

HIGH ESR

OUTPUT CAP

AVERAGE

COMP1

OUTPUT

LTC1429 • F07

Note that ESR in the flying caps will not cause the same

condition; in fact, it may actually improve the situation by

cutting the peak currents and lowering the amplitude of

the spike. More flying cap ESR is not necessarily better,

however; 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 LTC1429 will begin to diminish. For 0.1

µF flying

capacitors and typical 700kHz external clock, this gives a

maximum total series resistance of:

=

/ 0.1

µF = 7.14Ω

1

2

)

1

2

)

700kHz

Most of this resistance is already provided by the internal

switches in the LTC1429 (especially in tripler mode). More

than 1

Ω or 2Ω of ESR on the flying caps will start to affect

the regulation at maximum load.

RESISTOR SELECTION

Resistor selection is easy with the fixed output versions of

the LTC1429; 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

with respect to ground (Figure 4 shows reference connec-

tion). 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.

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 RO, R1 and

124k, 226k, 100k and 50k (see Figure 4) 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’s 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.

Table 3. Output Voltages Using the Internal Resistor Divider

PIN CONNECTIONS

OUTPUT VOLTAGE

–5.0V

– 4.5V

– 4.0V

– 3.5V

ADJ - R1

– 1.77V

ADJ - R0

– 1.38V

ADJ - GND

– 1.24V

There are some oddball output voltages available as well.

They are obtained by connecting ADJ to R0 or R1 and

shorting out some of the internal resistors. If one of them

gives you the output voltage you want, by all means use it!

LTC1429

GND

R1

6 (4)*

10 (5)*

11 (6)*

LTC1429CS

*(LTC1429CS8)

LTC1429 • F07

R2

(R1 + R2)

R2

ADJ

OUT

Figure 8. External Resistor Connections