LLT3837EFE datasheet(21/28 Pages) LINER

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LT3837

3837fa

APPLICATIONS INFORMATION

Short-Circuit Conditions

Loss of current limit is possible under certain conditions

such as an output short circuit. If the duty cycle exhib-

ited by the minimum on-time is greater than the ratio of

secondary winding voltage (referred-to-primary) divided

by input voltage, then peak current is not controlled at

the nominal value. It ratchets up cycle-by-cycle to some

higher level. Expressed mathematically, the requirement

to maintain short-circuit control is:

MIN

ON MIN

OSC

SEC

DS ON

()

•

• SSP

where:

tON(MIN) = primary side switch minimum on-time

ISC = short-circuit output current

Other variables as previously deﬁned.

Trouble is typically encountered only in applications with a

relatively high product of input voltage times secondary-

to-primary turns ratio and/or a relatively long minimum

switch on time. Additionally, several real world effects such

as transformer leakage inductance, AC winding losses, and

output switch voltage drop combine to make this simple

theoretical calculation a conservative estimate. Prudent

design evaluates the switcher for short-circuit protection

and adds any additional circuitry to prevent destruction.

Output Voltage Error Sources

The LT3837’s feedback sensing introduces additional

sources of errors. The following is a summary list.

The internal bandgap voltage reference sets the reference

voltage for the feedback ampliﬁer. The speciﬁcations detail

its variation.

The external feedback resistive divider ratio proportional

directly affects regulated voltage. Use 1% components.

Leakage inductance on the transformer secondary reduces

the effective secondary-to-feedback winding turns ratio

(NS/NF) from its ideal value. This increases the output volt-

age target by a similar percentage. Since secondary leakage

inductance is constant from part to part (with a tolerance)

adjust the feedback resistor ratio to compensate.

The transformer secondary current ﬂows through the im-

pedances of the winding resistance, synchronous MOSFET

RDS(ON) and output capacitor ESR. The DC equivalent

current for these errors is higher than the load current

because conduction occurs only during the converter’s

“off” time. So divide the load current by (1 – DC).

If the output load current is relatively constant, the feedback

resistive divider is used to compensate for these losses.

Otherwise, use the LT3837 load compensation circuitry

(see Load Compensation).

If multiple output windings are used, the ﬂyback winding

will have a signal that represents an amalgamation of all

these windings impedances. Take care that you examine

worst-case loading conditions when tweaking the volt-

ages.

Power MOSFET Selection

The power MOSFETs are selected primarily on the criteria

of “on” resistance RDS(ON), input capacitance, drain-to-

source breakdown voltage (BVDSS),maximumgatevoltage

(VGS) and maximum drain current (ID(MAX)).

For the primary-side power MOSFET, the peak current

is:

OUT

MAX

MIN

⎛

⎝⎜

⎞

⎠⎟

–

•

where X is peak-to-peak current ratio as deﬁned earlier.

For each secondary-side power MOSFET, the peak cur-

rent is:

OUT

MAX

MIN

⎛

⎝⎜

⎞

⎠⎟

–

•

Select a primary-side power MOSFET with a BVDSS greater

than:

DSS

LKG

IN MAX

OUT MAX

≥+

()

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