LTC4268IDKD-1#PBF【PDF 37/46 页】IC PD HIGH POWER W/CNTRL 32-DFN

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42681fc

LTC4268-1

applicaTions inForMaTion

max 100mV rating because of the minimum switch on

time blanking. If the voltage on V

SENSE

exceeds 205mV

after the minimum turn-on time, the SFST capacitor is

discharged, causing the discharge of the V

CMP

capacitor.

This then reduces the peak current on the next cycle and

will reduce overall stress in the primary switch.

Short-Circuit Conditions

Loss of current limit is possible under certain conditions

such as an output short circuit. If the duty cycle exhibited

by the minimum on time is greater than the ratio of

secondary winding voltage (referred-to-primary) divided

by input voltage, then peak current is not controlled at

the nominal value. It ratchets up cycle-by-cycle to some

higher level. Expressed mathematically, the requirement

to maintain short-circuit control is

MIN

= t

ON(MIN)

OSC

SEC

DS(ON)

(

)

where:

ON(MIN)

is the primary side switch minimum on-time

is the short-circuit output current

is the secondary-to-primary turns ratio (N

SEC

PRI

)

(Other variables as previously defined)

Trouble is typically encountered only in applications with a

relatively high product of input voltage times secondary to

primary turns ratio and/or a relatively long minimum switch

on time. Additionally, several real world effects such as

transformer leakage inductance, AC winding losses, and

output switch voltage drop combine to make this simple

theoretical calculation a conservative estimate. Prudent

design evaluates the switcher for short-circuit protection

and adds any additional circuitry to prevent destruction

for these losses.

Output Voltage Error Sources

The LTC4268-1s feedback sensing introduces additional

minor sources of errors. The following is a summary list.

" The internal bandgap voltage reference sets the reference

voltage for the feedback amplifier. The specifications

detail its variation.

" The external feedback resistive divider ratio directly

affects regulated voltage. Use 1% components.

" Leakage inductance on the transformer secondary

reduces the effective secondary-to-feedback winding

turns ratio (NS/NF) from its ideal value. This increases

the output voltage target by a similar percentage. Since

secondary leakage inductance is constant from part to

part (within a tolerance) adjust the feedback resistor

ratio to compensate.

" The transformer secondary current flows through the

impedances of the winding resistance, synchronous

MOSFET R

DS(ON)

and output capacitor ESR. The DC

equivalent current for these errors is higher than the

load current because conduction occurs only during

the converters off time. So divide the load current

by (1 DC).

If the output load current is relatively constant, the feedback

resistive divider is used to compensate for these losses.

Otherwise, use the LTC4268-1 load compensation circuitry.

(See Load Compensation.) If multiple output windings are

used, the flyback winding will have a signal that represents

an amalgamation of all these windings impedances. Take

care that you examine worst-case loading conditions when

tweaking the voltages.

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