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LT3512

3512f

APPLICATIONS INFORMATION

Table 2 and 3 show some recommended diodes and Zener

diodes.

Table 2. Recommended Zener Diodes

PART

VZENER

(V)

POWER

(W)

CASE

VENDOR

MMSZ5266BT1G

0.5

SOD-123

On Semi

MMSZ5270BT1G

0.5

SOD-123

CMHZ5266B

0.5

SOD-123

Central

Semiconductor

CMHZ5267B

0.5

SOD-123

BZX84J-68

0.5

SOD323F NXP

BZX100A

100

0.5

SOD323F

Table 3. Recommended Diodes

PART

I (A)

VREVERSE

(V)

VENDOR

DFLS1100

1.0

100

Diodes Inc.

DFLS1150

1.0

150

Leakage Inductance Blanking

When the power switch turns off, the flyback pulse ap-

pears. However, a finite time passes before the trans-

former primary-side voltage waveform approximately

represents the output voltage. Rise time on the SW node

and transformer leakage inductance cause the delay. The

leakage inductance also causes a very fast voltage spike

on the primary side of the transformer. The amplitude of

the leakage spike is largest when power switch current is

highest. Introduction of an internal fixed delay between

switch turn-off and the start of sampling provides im-

munity to the phenomena discussed above. The LT3512

sets internal blanking to 150ns. In certain cases leakage

inductance spikes last longer than the internal blanking,

but will not significantly affect output regulation.

Secondary Leakage Inductance

In addition to primary leakage inductance, secondary leak-

age inductance exhibits an important effect on application

design. Secondary leakage inductance forms an inductive

divider on the transformer secondary. The inductive divider

effectively reduces the size of the primary-referred flyback

pulse. The smaller flyback pulse results in a higher regulated

output voltage. The inductive divider effect of secondary

leakage inductance is load independent. RFB/RREF ratio

adjustments can accommodate this effect to the extent

secondary leakage inductance is a constant percentage

of mutual inductance (over manufacturing variations).

Winding Resistance Effects

Resistance in either the primary or secondary will reduce

overall efficiency (POUT/PIN). Good output voltage regula-

tion will be maintained independent of winding resistance

due to the boundary mode operation of the LT3512.

Bifilar Winding

A bifilar, or similar winding technique, is a good way to

minimize troublesome leakage inductances. However, re-

member that this will also increase primary-to-secondary

capacitance and limit the primary-to-secondary breakdown

voltage, so bifilar winding is not always practical. The

Linear Technology applications group is available and

extremely qualified to assist in the selection and/or design

of the transformer.

APPLICATION DESIGN CONSIDERATIONS

Iterative Design Process

The LT3512 uses a unique sampling scheme to regulate

the isolated output voltage. The use of this isolated scheme

requires a simple iterative process to choose feedback

resistors and temperature compensation. Feedback re-

sistor values and temperature compensation resistance

is heavily dependent on the application, transformer and

output diode chosen.

Once resistor values are fixed after iteration, the values

will produce consistent output voltages with the chosen

transformer and output diode. Remember, the turns ratio

of the transformer must be guaranteed within ±1%. The

transformer vendors mentioned in this data sheet can

build transformers to this specification.

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