LT1425CS datasheet(13/20 Pages) LINER

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LT1425

APPLICATIONS INFORMATION

degrades load regulation (at least before load compensa-

tion is employed).

Bifilar Winding

A bifilar or similar winding technique is a good way to

minimize troublesome leakage inductances. However,

remember that this will increase primary-to-secondary

capacitance and limit the primary-to-secondary break-

down voltage, so bifilar winding is not always practical.

Finally, the LTC Applications group is available to assist

in the choice and/or design of the transformer. Happy

Winding!

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 LT1425, 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 for Reference

Current.

User Programming Resistors

Output voltage is controlled by the ratio of RFB to RREF.

Both are user supplied external resistors. To the extent

that the resistor ratio differs from the ideal value, the

output voltage will be proportionally affected.

Schottky Diode Drop

The LT1425 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

signal! It then reverts to a potentially stable state whereby

the top of the leakage spike is the control point, and the

trailing edge of the leakage spike triggers the collapse

detect circuitry. This will typically reduce the output volt-

age abruptly to a fraction, perhaps between one-third to

two-thirds of its correct value. If load current is reduced

sufficiently, the system will snap back to normal opera-

tion. When using transformers with considerable leakage

inductance, it is important to exercise this worst-case

check for potential bistability:

1. Operate the prototype supply at maximum expected

load current.

2. Temporarily short circuit the output.

3. Observe that normal operation is restored.

If the output voltage is found to hang up at an abnormally

low value, the system has a problem. This will usually be

evident by simultaneously monitoring the VSW waveform

on an oscilloscope to observe leakage spike behavior

firsthand. A final note, the susceptibility of the system to

bistable behavior is somewhat a function of the load I/V

characteristics. A load with resistive, i.e., I = V/R behavior

is the most susceptible to bistability. Loads which exhibit

“CMOSsy”, i.e., I = V2/R behavior are less susceptible.

Secondary Leakage Inductance

In addition to the previously described effects of leakage

inductance in general, leakage inductance on the second-

ary in particular exhibits an additional phenomenon. It

forms an inductive divider on the transformer secondary,

that reduces the size of the primary-referred flyback pulse

used for feedback. This will increase the output voltage

target by a similar percentage. Note that unlike leakage

spike behavior, this phenomenon is load independent. To

the extent that the secondary leakage inductance is a

constant percentage of mutual inductance (over manufac-

turing variations), this can be accommodated by adjusting

the RFB/RREF resistor ratio.

Winding Resistance Effects

Resistance in either the primary or secondary will act to

reduce overall efficiency (POUT/PIN). Resistance in the

secondary increases effective output impedance which

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