LTC1736 datasheet(9/28 Pages) LINER

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LTC1736

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 1.2

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 condi-

tions 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 VFB drops below 0.6V, the

buffered ITH input to the current comparator is gradually

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

current to about 1/4 of its maximum value.

Low Current Operation

The LTC1736 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). During

Burst Mode operation, if the error amplifier drives the ITH

voltage below 0.86V, the buffered ITH input to the current

comparator 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 operation 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

current, 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

condition. If the condition lasts for a long enough period

as determined by the size of the 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

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

current shortens the soft-start period but also prevents net

discharge 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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