10

LTC3803-3

38033fa

APPLICATIO S I FOR ATIO

Many LTC3803-3 application circuits can be derived from

the topology shown in Figure 2.

The LTC3803-3 itself imposes no limits on allowed power

external power components. The key factors are: Q1’s

LTC3803-3

GND

NGATE

SENSE

5

R3

6

4

1

2

3

D1

D2

•

•

38033 F02

R1

R2

Q1

T1

•

Figure 2. Typical LTC3803-3 Application Circuit

TRANSFORMER DESIGN CONSIDERATIONS

Transformer specification and design is perhaps the most

critical part of applying the LTC3803-3 successfully. In

addition to the usual list of caveats dealing with high fre-

quency power transformer design, the following should

prove useful.

Turns Ratios

Due to the use of the external feedback resistor divider

ratio to set output voltage, the user has relative freedom in

selecting transformer turns ratio to suit a given applica-

tion. Simple ratios of small integers, e.g., 1:1, 2:1, 3:2, etc.

can be employed which yield more freedom in setting total

turns and mutual inductance. Simple integer turns ratios

also facilitate the use of “off-the-shelf” configurable trans-

formers such as the Coiltronics VERSA-PAC

applications with high input to output voltage ratios. For

example, if a 6-winding VERSA-PAC is used with three

windings in series on the primary and three windings in

parallel on the secondary, a 3:1 turns ratio will be achieved.

Turns ratio can be chosen on the basis of desired duty

cycle. However, remember that the input supply voltage

plus the secondary-to-primary referred version of the

flyback pulse (including leakage spike) must not exceed

the allowed external MOSFET breakdown rating.

Leakage Inductance

Transformer leakage inductance (on either the primary or

secondary) causes a voltage spike to occur after the

output switch (Q1) turn-off. This is increasingly promi-

nent at higher load currents, where more stored energy

must be dissipated. In some cases a “snubber” circuit will

be required to avoid overvoltage breakdown at the

MOSFET’s drain node. Application Note 19 is a good

reference on snubber design.

A bifilar or similar winding technique is a good way to

minimize troublesome leakage inductances. However,

remember that this will limit the primary-to-secondary

breakdown voltage, so bifilar winding is not always

practical.

SELECTING FEEDBACK RESISTOR DIVIDER VALUES

The regulated output voltage is determined by the resistor

R

VV

V

R

OUT

2

08

08

1

=

–.

.

•

Choose resistance values for R1 and R2 to be as large as

possible in order to minimize any efficiency loss due to the

good rule of thumb is to choose R1 to be 80k or less.