11

LTC1649

used to power external circuitry, but any additional current

drive the external MOSFETs. Circuits with small external

MOSFETs can draw as much as 20mA or 30mA from

rent circuits with large or multiple external MOSFETs may

more limited in its abilities, and should not be connected

charge from the charge pump and actually degrade perfor-

mance.

Compensation and Transient Response

The LTC1649 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 5). 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.

Output transient response is set by three major factors: the

time constant of the inductor and the output capacitor, the

ESR of the output capacitor, and the loop compensation

components. The first two factors usually have much

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-

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.

Soft Start and Current Limit

The LTC1649 includes a soft start circuit at the SS pin; this

circuit is used both for initial start-up and during current

limit operation. 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

controlling the duty cycle (Figure 6). The LTC1649 will

begin to operate at low duty cycle as the SS pin rises to

duty cycle will increase until the error amplifier takes over

and begins to regulate the output. When SS reaches 1V

switch shorts the SS pin to GND during shutdown.

LTC1649

COMP

1659 F05

C1

Figure 5. Compensation Pin Hook-Up

Figure 6. Soft Start Clamps COMP Pin

1649 F06

COMP

SS

LTC1649

12

µA

FB