LTC1735CS datasheet(9/32 Pages) LINER

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LTC1735

OPERATIO

(Refer to Functional Diagram)

Main Control Loop

The LTC1735 uses a constant frequency, current mode

step-down architecture. During normal operation, the top

MOSFET is turned on each cycle when the oscillator sets

the RS latch and turned off when the main current com-

parator I1 resets the RS latch. The peak inductor current at

which I1 resets the RS latch is controlled by the voltage on

Pin 3 (ITH), which is the output of error amplifier EA. Pin 6

(VOSENSE), described in the pin functions, allows EA to

receive an output feedback voltage VFB from an external

resistive divider. When the load current increases, it

causes a slight decrease in VFB relative to the 0.8V refer-

ence, which in turn causes the ITH voltage to increase until

the average inductor current matches the new load cur-

rent. While the top MOSFET is off, the bottom MOSFET is

turned on until either the inductor current starts to reverse,

as indicated by current comparator I2, or the beginning of

the next cycle.

The top MOSFET driver is powered from a floating boot-

strap capacitor CB. This capacitor is normally recharged

from INTVCC through an external diode when the top

MOSFET is turned off. As VIN decreases towards VOUT, the

converter will attempt to turn on the top MOSFET continu-

ously (“dropout’’). A dropout counter detects this condi-

tion and forces the top MOSFET to turn off for about 500ns

every tenth cycle to recharge the bootstrap capacitor.

The main control loop is shut down by pulling Pin 2

(RUN/SS) low. Releasing RUN/SS allows an internal 1.2

µA

current source to charge soft-start capacitor CSS. When

CSS reaches 1.5V, the main control loop is enabled with the

ITH voltage clamped at approximately 30% of its maximum

value. As CSS continues to charge, ITH is gradually re-

leased allowing normal operation to resume. If VOUT has

not reached 70% of its final value when CSS has charged

to 4.1V, latchoff can be invoked as described in the

Applications Information section.

The internal oscillator can be synchronized to an external

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

between 90% and 130% of its nominal rate set by capaci-

tor COSC.

An overvoltage comparator, OV, guards against transient

overshoots (>7.5%) as well as other more serious

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 condition is cleared.

Foldback current limiting for an output shorted to ground

is provided by amplifier A. As VOSENSE drops below 0.6V,

the buffered ITH input to the current comparator is gradu-

ally pulled down to a 0.86V clamp. This reduces peak

inductor current to about 1/4 of its maximum value.

Low Current Operation

The LTC1735 has three low current modes controlled by

the FCB pin. Burst Mode operation is selected when the

FCB pin is above 0.8V (typically tied to INTVCC). In Burst

Mode operation, if the error amplifier drives the ITH voltage

below 0.86V, the buffered ITH input to the current com-

parator will be clamped at 0.86V. The inductor current

peak is then held at approximately 20mV/RSENSE (about

1/4 of maximum output current). If ITH then drops below

0.5V, the Burst Mode comparator B will turn off both

MOSFETs to maximize efficiency. The load current will be

supplied solely by the output capacitor until ITH rises

above the 60mV hysteresis of the comparator and switch-

ing is resumed. Burst Mode operation is disabled by

comparator F when the FCB pin is brought below 0.8V.

This forces continuous operation and can assist second-

ary winding regulation.

When the FCB pin is driven by an external oscillator, a low

noise cycle-skipping mode is invoked and the internal

oscillator is synchronized to the external clock by com-

parator C. In this mode the 25% minimum inductor

current clamp is removed, providing constant frequency

discontinuous operation over the widest possible output

current range. This constant frequency operation is not

quite as efficient as Burst Mode operation, but provides a

lower noise, constant frequency spectrum.

The FCB pin is tied to ground when forced continuous

operation is desired. This is the least efficient mode, but is

desirable in certain applications. The output can source or

sink current in this mode. When sinking current while in

forced continuous operation, current will be forced back

into the main power supply potentially boosting the input

supply to dangerous voltage levels—BEWARE.

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