LTC1735E Просмотр технического описания (PDF) - Linear Technology

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LTC1735E

High Efficiency Synchronous Step-Down Switching Regulator

Linear Technology 

LTC1735

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OPERATIO (Refer to Functional Diagram)

The main control loop is shut down by pulling Pin 2

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

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.

Tying the FCB pin to ground enables forced continuous

operation. 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.

Foldback Current, Short-Circuit Detection and

Short-Circuit Latchoff

The RUN/SS capacitor, CSS, is used initially to limit the

inrush current of the switching regulator. After the con-

troller has been started and been given adequate time to

charge up the output capacitors and provide full load cur-

rent, CSS is used as a short-circuit time-out circuit. If the

output voltage falls to less than 70% of its nominal output

voltage, CSS begins discharging on the assumption that

the output is in an overcurrent and/or short-circuit condi-

tion. If the condition lasts for a long enough period as

determined by the size of CSS, the controller will be shut

down until the RUN/SS pin voltage is recycled. This built-

in latchoff can be overridden by providing a current >5µA

at a compliance of 5V to the RUN/SS pin. This current

shortens the soft-start period but also prevents net dis-

charge of CSS during an overcurrent and/or short-circuit

condition. Foldback current limiting is activated when the

output voltage falls below 70% of its nominal level whether

or not the short-circuit latchoff circuit is enabled.

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