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LT1725

1725f

Capacitor C1 should then be made large enough to avoid

the relaxation oscillatory behavior described above. This

is complicated to determine theoretically as it depends on

the particulars of the secondary circuit and load behavior.

Empirical testing is recommended. (Use of the optional

soft-start function will lengthen the power-up timing and

require a correspondingly larger value for C1.)

A further note—certain users may wish to utilize the

general functionality of the LT1725, but may have an

available input voltage significantly lower than, say, 48V.

If this input voltage is within the allowable VCC range, i.e.,

perhaps 20V maximum, the internal wide hysteresis range

UVLO function becomes counterproductive. In such cases

it is simply better to operate the LT1725 directly from the

available DC input supply. The LT1737 is identical to the

LT1725, with the exception that it lacks the internal wide

hysteresis UVLO function. It is therefore designed to

operate directly from DC input supplies in the range of

4.5V to 20V. See the LT1737 data sheet for further

information.

FREQUENCY COMPENSATION

Loop frequency compensation is performed by connect-

ing a capacitor from the output of the error amplifier (VC

pin) to ground. An additional series resistor, often re-

quired in traditional current mode switcher controllers, is

usually not required and can even prove detrimental. The

phase margin improvement traditionally offered by this

extra resistor will usually be already accomplished by the

nonzero secondary circuit impedance, which adds a “zero”

to the loop response.

In further contrast to traditional current mode switchers,

VC pin ripple is generally not an issue with the LT1725. The

dynamic nature of the clamped feedback amplifier forms

an effective track/hold type response, whereby the VC

voltage changes during the flyback pulse, but is then “held”

during the subsequent “switch on” portion of the next

cycle. This action naturally holds the VC voltage stable

during the current comparator sense action (current mode

switching).

OUTPUT VOLTAGE ERROR SOURCES

Conventional nonisolated switching power supply ICs

typically have only two substantial sources of output

voltage error: the internal or external resistor divider

network that connects to VOUT and the internal IC refer-

ence. The LT1725, which senses the output voltage in both

a dynamic and an isolated manner, exhibits additional

potential error sources to contend with. Some of these

errors are proportional to output voltage, others are fixed

in an absolute millivolt sense. Here is a list of possible

error sources and their effective contribution.

Internal Voltage Reference

The internal bandgap voltage reference is, of course,

imperfect. Its error, both at 25

°C and over temperature is

already included in the specifications.

User Programming Resistors

Output voltage is controlled by the user-supplied feedback

resistor divider ratio. To the extent that the resistor ratio

differs from the ideal value, the output voltage will be

proportionally affected. Highest accuracy systems will

demand 1% components.

Schottky Diode Drop

The LT1725 senses the output voltage from the trans-

former primary side during the flyback portion of the cycle.

This sensed voltage therefore includes the forward drop,

VF, of the rectifier (usually a Schottky diode). The nominal

VF of this diode should therefore be included in feedback

resistor divider calculations. Lot to lot and ambient tem-

perature variations will show up as output voltage shift/

drift.

Secondary Leakage Inductance

Leakage inductance on the transformer secondary re-

duces the effective secondary-to-third winding turns ratio

(NS/NT) from its ideal value. This will increase the output

voltage target by a similar percentage. To the extent that

secondary leakage inductance is constant from part to

part, this can be accommodated by adjusting the feedback

resistor ratio.

APPLICATIO S I FOR ATIO

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