LTC1429CS View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC1429CS

Clock-Synchronized Switched Capacitor Regulated Voltage Inverter

Linear Technology 

LTC1429

APPLICATIONS INFORMATION

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

as if a resistor of value CESR × (IPK/IAVE)Ω was placed in series

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

should be as low as possible or smaller value high frequency

bypass (typically a 0.1µF ceramic) should be added in parallel

with the output capacitor.

CLOCK

LOW ESR

OUTPUT CAP

HIGH ESR

OUTPUT CAP

VOUT

VOUT

VSET

VOUT

AVERAGE

COMP1

OUTPUT

VSET

VOUT

AVERAGE

COMP1

OUTPUT

LTC1429 • F07

Figure 7. Output Ripple with Low and High ESR Caps

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 0.1µF flying

capacitors and typical 700kHz external clock, this gives a

maximum total series resistance of:

) ) 1

2

tCLK

CFLY

=

1

2

1

700kHz

/ 0.1µF = 7.14Ω

Most of this resistance is already provided by the internal

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

than 1Ω 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

respect to VOUT (Figure 8). The LTC1429 uses a positive

reference with respect to VOUT, not a negative reference

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.

6 (4)*

GND

LTC1429

R1

10 (5)*

ADJ

R2

11 (6)*

OUT

LTC1429CS

*(LTC1429CS8)

(R1 + R2)

VOUT = –1.24V R2

LTC1429 • F07

Figure 8. External Resistor Connections

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

RADJ pins (Table 3). The internal resistors are roughly

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

ADJ - RADJ

ADJ - RADJ, RO - GND

ADJ - RADJ, R1 - RO

ADJ - RADJ, R1-GND

ADJ - R1

– 5.0V

– 4.5V

– 4.0V

– 3.5V

– 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!

10

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