LTC1775CS datasheet(16/24 Pages) LINER

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LTC1775

In addition to providing a logic input to force continuous

operation, the FCB pin provides a means to regulate a

flyback winding output. It can force continuous synchro-

nous operation when needed by the flyback winding,

regardless of the main output load.

The secondary output voltage VSEC is normally set as

shown in Figure 5a by the turns ratio N of the transformer:

VSEC ≅ (N + 1)VOUT

However, if the controller goes into Burst Mode operation

and halts switching due to a light main load current, then

VSEC will droop. An external resistor divider from VSEC to

the FCB pin sets a minimum voltage VSEC(MIN):

SEC MIN

()

≅+









119

If VSEC drops below this level, the FCB voltage forces

continuous operation until VSEC is again above its

minimum.

Minimum On-Time Considerations

Minimum on-time tON(MIN) is the smallest amount of time

that the LTC1775 is capable of turning the top MOSFET on

and off again. It is determined by internal timing delays and

the amount of gate charge required to turn on the top

MOSFET. Low duty cycle applications may approach this

minimum on-time limit and care should be taken to ensure

that:

ON MIN

OUT

()

()( )

If the duty cycle falls below what can be accommodated by

the minimum on-time, the LTC1775 will begin to skip

cycles. The output voltage will continue to be regulated,

but the ripple current and ripple voltage will increase.

The minimum on-time for the LTC1775 is generally about

0.5

µs. However, as the peak sense voltage (IL(PEAK) •

RDS(ON)) decreases, the minimum on-time gradually

increases up to about 0.7

µs. This is of particular concern

in forced continuous applications with low ripple current

at light loads. If the duty cycle drops below the minimum

on-time limit in this situation, a significant amount of

Run/Soft Start Function

The RUN/SS pin is a dual purpose pin that provides a soft

start function and a means to shut down the LTC1775. Soft

start reduces surge currents from VIN by gradually in-

creasing the controller’s current limit ITH(MAX). This pin

can also be used for power supply sequencing.

Pulling the RUN/SS pin below 1.4V puts the LTC1775 into

a low quiescent current shutdown (IQ< 30µA). This pin can

be driven directly from logic as shown in Figure 9. Releas-

ing the RUN/SS pin allows an internal 3

µA current source

to charge up the soft-start capacitor CSS. If RUN/SS has

been pulled all the way to ground there is a delay before

starting of approximately:

F C

DELAY









=µ

()

When the voltage on RUN/SS reaches 1.4V the LTC1775

begins operating with a clamp on ITH at 0.8V. As the

voltage on RUN/SS increases to approximately 3.1V, the

clamp on ITH is raised until its full 2.4V range is restored.

This takes an additional 0.5s/

µF. During this time the load

current will be folded back to approximately 80mV/RDS(ON)

until the output reaches half of its final value.

Diode D1 in Figure 9 reduces the start delay while allowing

CSS to charge up slowly for the soft start function. This

diode and CSS can be deleted if soft start is not needed. The

RUN/SS pin has an internal 6V zener clamp (See Func-

tional Diagram).

Figure 9. RUN/SS Pin Interfacing

3.3V

OR 5V

RUN/SS

CSS

1775 F09

RUN/SS

CSS

FCB Pin Operation

When the FCB pin drops below its 1.19V threshold,

continuous synchronous operation is forced. In this case,

the top and bottom MOSFETs continue to be driven

regardless of the load on the main output. Burst Mode

operation is disabled and current reversal under light

loads is allowed in the inductor.

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