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LT1737

1737f

VIN

UVLO

VIN

UVLO

100pF

VIN

UVLO

R1/2

1737 F06

100pF

APPLICATIO S I FOR ATIO

Figure 6

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 LT1737. 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 LT1737, 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 LT1737 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 temperature 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.

(6c) Recommended Topology to

Filter High Frequency Ripple

(6b) Filter Capacitor

Directly on UVLO Note

(6a) “Standard” UVLO

Divider Topology

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