LT1425CS datasheet(12/20 Pages) LINER

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LT1425

APPLICATIONS INFORMATION

amount when output switch current is zero. Please refer

to Figure 1 which shows nominal reference current shift

at zero load for a range of ROCOMP values. Example: for

a load compensation resistor of 12k, the graph indi-

cates a 1.0% shift in reference current. The RFB resistor

value should be adjusted down by about 1.0% to

restore the original target output voltage.

integers, e.g., 1:1, 2:1, 3:2, etc. can be employed which

yield more freedom in setting total turns and mutual

inductance. Turns ratio can then 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 output switch breakdown rating.

Leakage Inductance

Transformer leakage inductance (on either the primary or

secondary) causes a spike after output switch turn-off.

This is increasingly prominent at higher load currents

where more stored energy must be dissipated. In many

cases a “snubber” circuit will be required to avoid over-

voltage breakdown at the output switch node. LTC’s

Application Note 19 is a good reference on snubber

design.

In situations where the flyback pulse extends beyond the

enable delay time, the output voltage regulation will be

affected to some degree. It is important to realize that the

feedback system has a deliberately limited input range,

roughly

±50mV referred to the RREF node, and this works

to the user’s advantage in rejecting large, i.e., higher

voltage leakage spikes. In other words, once a leakage

spike is several volts in amplitude, a further increase in

amplitude has little effect on the feedback system. So the

user is generally advised to arrange the snubber circuit to

clamp at as high a voltage as comfortably possible,

observing switch breakdown, such that leakage spike

duration is as short as possible.

As a rough guide, total leakage inductances of several

percent (of mutual inductance) or less may require a

snubber, but exhibit little to no regulation error due to

leakage spike behavior. Inductances from several percent

up to perhaps ten percent cause increasing regulation

error.

Severe leakage inductances in the double digit percentage

range should be avoided if at all possible as there is a

potential for abrupt loss of control at high load current.

This curious condition potentially occurs when the leak-

age spike becomes such a large portion of the flyback

waveform that the processing circuitry is fooled into

thinking that the leakage spike itself is the real flyback

In less critical applications, or when output current

remains relatively constant, the load compensation func-

tion may be deemed unnecessary. In such cases, a

reduced component solution may be obtained as follows:

Leave the ROCOMP node open (ROCOMP = ∞), and replace

the filter capacitor normally on the RCCOMP node with a

short to ground.

TRANSFORMER DESIGN CONSIDERATIONS

Transformer specification and design is perhaps the most

critical part of applying the LT1425 successfully. In addi-

tion to the usual list of caveats dealing with high frequency

isolated power supply transformer design, the following

information should prove useful.

Turns Ratio

Note that due to the use of an RFB/RREF resistor ratio to set

output voltage, the user has relative freedom in selecting

transformer turns ratio to suit a given application. In other

words, “screwball” turns ratios like “1.736:1.0” can scru-

pulously be avoided! In contrast, simpler ratios of small

ROCOMP (kΩ)

100

1000

1425 F01

Figure 1

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