LTC3104 datasheet(10/26 Pages) LINER

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LTC3630

3630fb

An internal 0.8ms soft-start function limits the ramp rate

of the output voltage on start-up to prevent excessive input

supply droop. If a longer ramp time and consequently less

supply droop is desired, a capacitor can be placed from

the SS pin to ground. The 5μA current that is sourced

out of this pin will create a smooth voltage ramp on the

capacitor. If this ramp rate is slower than the internal

0.8ms soft-start, then the output voltage will be limited

by the ramp rate on the SS pin instead. The internal and

external soft-start functions are reset on start-up and after

an undervoltage event on the input supply.

The peak inductor current is not limited by the internal or

external soft-start functions; however, placing a capacitor

from the ISET pin to ground does provide this capability.

Peak Inductor Current Programming

The peak current comparator nominally limits the peak

inductor current to 1.2A. This peak inductor current can

be adjusted by placing a resistor from the ISET pin to

ground. The 5μA current sourced out of this pin through

the resistor generates a voltage that adjusts the peak cur-

rent comparator threshold.

During sleep mode, the current sourced out of the ISET pin

is reduced to 1μA. The ISET current is increased back to 5μA

on the first switching cycle after exiting sleep mode. The

ISET current reduction in sleep mode, along with adding

a filtering capacitor, CISET, from the ISET pin to ground,

provides a method of reducing light load output voltage

ripple at the expense of lower efficiency and slightly de-

graded load step transient response.

For applications requiring higher output current, the

LTC3630 provides a feedback comparator output pin (FBO)

for combining the output current of multiple LTC3630s. By

connecting the FBO pin of a “master” LTC3630 to the VFB

pin of one or more “slave” LTC3630s, the output currents

can be combined to source much more than 500mA.

OPERATION

Dropout Operation

When the input supply decreases toward the output sup-

ply, the duty cycle increases to maintain regulation. The

P-channel MOSFET top switch in the LTC3630 allows the

duty cycle to increase all the way to 100%. At 100% duty

cycle, the P-channel MOSFET stays on continuously, pro-

viding output current equal to the peak current, which can

be greater than 1A. The power dissipation of the LTC3630

can increase dramatically during dropout operation espe-

cially at input voltages less than 10V. The increased power

dissipation is due to higher potential output current and

increased P-channel MOSFET on-resistance. See the Ther-

mal Considerations section of the Applications Information

for a detailed example.

Input Voltage and Overtemperature Protection

When using the LTC3630, care must be taken not to

exceed any of the ratings specified in the Absolute Maxi-

mum Ratings section. As an added safeguard, however,

the LTC3630 incorporates an overtemperature shutdown

feature. If the junction temperature reaches approximately

180°C, the LTC3630 will enter thermal shutdown mode.

Both power switches will be turned off and the SW node

will become high impedance. After the part has cooled

below 160°C, it will restart. The overtemperature level is

not production tested.

The LTC3630 can provide a programmable undervoltage

lockout which can also serve as a precise input voltage

monitor by using a resistive divider from VIN to GND with

the tap connected to the RUN pin. Switching is enabled

when the RUN pin voltage exceeds 1.21V and is disabled

when dropping below 1.1V. Pulling the RUN pin below

700mV forces a low quiescent current shutdown (5μA).

Furthermore, if the input voltage falls below 3.5V typi-

cal (3.7V maximum), an internal undervoltage detector

disables switching.

When switching is disabled, the LTC3630 can safely sus-

tain input voltages up to the absolute maximum rating of

70V. Input supply undervoltage events trigger a soft-start

reset, which results in a graceful recovery from an input

supply transient.

(Refer to Block Diagram)

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