11

LTC1430

pensation components. In general, a smaller value induc-

tor will improve transient response at the expense of ripple

and inductor core saturation rating. Minimizing output

capacitor ESR will also help optimize output transient

response. See Input and Output Capacitors for more

information.

Compensation and Transient Response

The LTC1430 voltage feedback loop is compensated at the

amplifier. The loop can generally be compensated prop-

erly with an RC network from COMP to GND and an

additional small C from COMP to GND (Figure 8). Loop

stability is affected by inductor and output capacitor

values and by other factors. Optimum loop response can

be obtained by using a network analyzer to find the loop

poles and zeros; nearly as effective and a lot easier is to

looks right with an output load step. Table 1 shows

recommended compensation components for 5V to 3.3V

applications based on the inductor and output capacitor

values. The values were calculated using multiple paral-

leled 330

µF AVX TPS series surface mount tantalum

capacitors as the output capacitor.

Table 1. Recommended Compensation Network for 5V to 3.3V

Application Using Multiple 330

µF AVX Output Capacitors

L1 (

µH)

C1 (pF)

1

990

1.8

0.022

820

1

1980

3.6

0.01

470

1

4950

9.1

0.0047

150

1

9900

18

0.0022

82

2.7

990

3.6

0.01

470

2.7

1980

7.5

0.0047

220

2.7

4950

18

0.0022

82

2.7

9900

39

0.001

39

5.6

990

9.1

0.0047

150

5.6

1980

18

0.0022

82

5.6

4950

47

820pF

33

5.6

9900

91

470pF

15

10

990

18

0.0022

82

10

1980

39

0.001

39

10

4950

91

470pF

15

10

9900

180

220pF

10

Output transient response is set by three major factors: the

time constant of the inductor and the output capacitor, the

more impact on overall transient recovery time than the

third; unless the loop compensation is way off, more

improvement can be had by optimizing the inductor and

the output capacitor than by fiddling with the loop com-

LTC1430

COMP

GND

C1

Figure 8. Compensation Pin Hook-Up

Soft-Start and Current Limit

The 16-lead versions of the LTC1430 include a soft-start

circuit at the SS pin; this circuit is used both for initial start-

up and during current limit operation. The soft-start and

current limit circuitry is disabled in 8-lead versions. SS

requires an external capacitor to GND with the value

determined by the required soft-start time. An internal

12

µA current source is included to charge the external

capacitor. Soft-start functions by clamping the maximum

voltage that the COMP pin can swing to, thereby control-

ling the duty cycle (Figure 9). The LTC1430 will begin to

operate at low duty cycle as the SS pin rises to about 2V

increase until the error amplifier takes over and begins to

LTC1430 will be in full operation. An internal switch shorts

the SS pin to GND during shutdown.

The LTC1430 detects the output current by watching the

state, M1 has a known resistance; by calculating back-

APPLICATIO S I FOR ATIO