LTC4263 datasheet(30/42 Pages) LINER

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LTC4278

4278fc

APPLICATIONS INFORMATION

trace length and area to minimize stray capacitance and

potential noise pick-up.

Youcansynchronizetheoscillatorfrequencytoanexternal

frequency. This is done with a signal on the SYNC pin.

Set the LTC4278 frequency 10% slower than the desired

external frequency using the OSC pin capacitor, then use

a pulse on the SYNC pin of amplitude greater than 2V

and with the desired frequency. The rising edge of the

SYNC signal initiates an OSC capacitor discharge forcing

primaryMOSFEToff(PGvoltagegoeslow).Iftheoscillator

frequency is much different from the sync frequency,

problemsmayoccurwithslopecompensationandsystem

stability.Also,keepthesyncpulsewidthgreaterthan500ns.

Selecting Timing Resistors

There are three internal “one-shot” times that are

programmed by external application resistors: minimum

on-time, enable delay time and primary MOSFET turn-on

delay. These are all part of the isolated flyback control

technique, and their functions are previously outlined in

theTheoryofOperationsection.Thefollowinginformation

should help in selecting and/or optimizing these timing

values.

Minimum Output Switch On-Time (tON(MIN))

Minimumon-timeistheprogrammableperiodduringwhich

current limit is blanked (ignored) after the turn-on of the

primary-sideswitch.Thisimprovesregulatorperformance

by eliminating false tripping on the leading edge spike in

the switch, especially at light loads. This spike is due to

both the gate/source charging current and the discharge

of drain capacitance. The isolated flyback sensing requires

a pulse to sense the output. Minimum on-time ensures

that the output switch is always on a minimum time and

that there is always a signal to close the loop.

TheLTC4278doesnotemploycycleskippingatlightloads.

Therefore, minimum on-time along with synchronous

rectification sets the switch over to forced continuous

mode operation.

The tON(MIN) resistor is set with the following equation

RtON(MIN) kW

( )=

tON(MIN) ns

( )−104

1.063

Keep RtON(MIN) greater than 70k. A good starting value

is 160k.

Enable Delay Time (ENDLY)

Enabledelaytimeprovidesaprogrammabledelaybetween

turn-off of the primary gate drive node and the subsequent

enablingofthefeedbackamplifier.Asdiscussedearlier,this

delay allows the feedback amplifier to ignore the leakage

inductance voltage spike on the primary side. The worst-

case leakage spike pulse width is at maximum load condi-

tions. So, set the enable delay time at these conditions.

While the typical applications for this part use forced

continuous operation, it is conceivable that a secondary-

side controller might cause discontinuous operation at

light loads. Under such conditions, the amount of energy

stored in the transformer is small. The flyback waveform

becomes “lazy” and some time elapses before it indicates

the actual secondary output voltage. The enable delay time

should be made long enough to ignore the “irrelevant”

portion of the flyback waveform at light loads.

Even though the LTC4278 has a robust gate drive, the gate

transition time slows with very large MOSFETs. Increase

delay time as required when using such MOSFETs.

Theenabledelayresistorissetwiththefollowingequation:

RENDLY kW

( )=

tENDLY ns

( )−30

2.616

Keep RENDLYgreaterthan40k.Agoodstartingpointis56k.

Primary Gate Delay Time (PGDLY)

Primary gate delay is the programmable time from the

turn-off of the synchronous MOSFET to the turn-on of the

primary-side MOSFET. Correct setting eliminates overlap

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