LTC1775IGN datasheet(8/24 Pages) LINER

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LTC1775

Main Control Loop

The LTC1775 is a constant frequency, current mode

controller for DC/DC step-down converters. In normal

operation, the top MOSFET is turned on when the RS latch

is set by the on-chip oscillator and is turned off when the

current comparator I1 resets the latch. While the top

MOSFET is turned off, the bottom MOSFET is turned on

until either the inductor current reverses, as determined

by the current reversal comparator I2, or the next cycle

begins. Inductor current is measured by sensing the VDS

potential across the conducting MOSFET. The output of

the appropriate sense amplifier (TA or BA) is selected by

the switch logic and applied to the current comparator.

The voltage on the ITH pin sets the comparator threshold

corresponding to peak inductor current. The error ampli-

fier EA adjusts this voltage by comparing the feedback

signal VFB from the output voltage with the internal 1.19V

reference. The VPROG pin selects whether the feedback

voltage is taken directly from the VOSENSE pin or is derived

from an on-chip resistive divider. When the load current

increases, it causes a drop in the feedback voltage relative

to the reference. The ITH voltage then rises until the

average inductor current again matches the load current.

The internal oscillator can be synchronized to an external

clock applied to the SYNC pin and can lock to a frequency

between 100% and 150% of its nominal 150kHz rate.

When the SYNC pin is left open, it is pulled low internally

and the oscillator runs at its normal rate. If this pin is taken

above 1.2V, the oscillator will run at its maximum 225kHz

rate.

Pulling the RUN/SS pin low forces the controller into its

shutdown state and turns off both MOSFETs. Releasing

the RUN/SS pin allows an internal 3

µA current source to

charge up an external soft start capacitor CSS. When this

voltage reaches 1.4V, the controller begins switching, but

with the ITH voltage clamped at approximately 0.8V. As

CSS continues to charge, the clamp is raised until full range

operation is restored.

The top MOSFET driver is powered from a floating boot-

strap capacitor CB. This capacitor is normally recharged

from INTVCC through a diode DB when the top MOSFET is

turned off. As VIN decreases towards VOUT, the converter

will attempt to turn on the top MOSFET continuously

(‘’dropout’’). A dropout counter detects this condition and

forces the top MOSFET to turn off for about 500ns every

tenth cycle to recharge the bootstrap capacitor.

An overvoltage comparator OV guards against transient

overshoots and other conditions that may overvoltage the

output. In this case, the top MOSFET is turned off and the

bottom MOSFET is turned on until the overvoltage condi-

tion is cleared.

Foldback current limiting for an output shorted to ground

is provided by a transconductance amplifer CL. As VFB

drops below 0.6V, the buffered ITH input to the current

comparator is gradually pulled down to a 0.95V clamp.

This reduces peak inductor current to about one fifth of its

maximum value.

Low Current Operation

The LTC1775 is capable of Burst Mode operation at low

load currents. If the error amplifier drives the ITH voltage

below 0.95V, the buffered ITH input to the current com-

parator will remain clamped at 0.95V. The inductor current

peak is then held at approximately 60mV/RDS(ON)(TOP). If

ITH then drops below 0.5V, the Burst Mode comparator B

will turn off both MOSFETs. The load current will be

supplied solely by the output capacitor until ITH rises

above the 50mV hysteresis of the comparator and switch-

ing is resumed. Burst Mode operation is disabled by

comparator F when the FCB pin is brought below 1.19V.

This forces continuous operation and can assist second-

ary winding regulation.

INTVCC/EXTVCC Power

Power for the top and bottom MOSFET drivers and most

of the internal circuitry of the LTC1775 is derived from the

INTVCC pin. When the EXTVCC pin is left open, an internal

5.2V low dropout regulator supplies the INTVCC power

from VIN. If EXTVCC is raised above 4.7V, the internal

regulator is turned off and an internal switch connects

EXTVCC to INTVCC. This allows a high efficiency source,

such as the primary or a secondary output of the converter

itself, to provide the INTVCC power.

OPERATIO

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