LT1425CS datasheet(14/20 Pages) LINER

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LT1425

APPLICATIONS INFORMATION

“collapse,” thereby supporting operation well into discon-

tinuous mode. Nevertheless, there still remain constraints

to ultimate low load operation. They relate to the minimum

switch ON time and the minimum enable time. Discontinu-

ous mode operation will be assumed in the following

theoretical derivations.

As outlined in the Operation section, the LT1425 utilizes a

minimum output switch ON time, tON. This value can be

combined with expected VIN and switching frequency to

yield an expression for minimum delivered power.

)) f

LPRI

)

Min Power =

(VIN • tON)2

= (VOUT)(IOUT)

This expression then yields a minimum output current

constraint:

)) f

(LPRI)(VOUT)

)

IOUT(MIN) =

where,

f = Switching frequency (nominally 285kHz)

LPRI = Transformer primary side inductance

VIN = Input voltage

VOUT = Output voltage

tON = Output switch minimum ON time

(VIN • tON)2

An additional constraint has to do with the minimum

enable time. The LT1425 derives its output voltage infor-

mation from the flyback pulse. If the internal minimum

enable time pulse extends beyond the flyback pulse, loss

of regulation will occur. The onset of this condition can be

determined by setting the width of the flyback pulse equal

to the sum of the flyback enable delay, tED, plus the

minimum enable time, tEN. Minimum power delivered to

the load is then:

)) f

LSEC

)

Min Power =

[VOUT • (tEN + tED)]2

= (VOUT)(IOUT)

which yields a minimum output constraint:

VF of this diode should therefore be included in RFB

calculations. Lot-to-lot and ambient temperature varia-

tions will show up as output voltage shift/drift.

Secondary Leakage Inductance

Leakage inductance on the transformer secondary

reduces the effective primary-to-secondary turns ratio

(NP/NS) 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 RFB to

RREF resistor ratio.

Output Impedance Error

An additional error source is caused by transformer sec-

ondary current flow through the real life nonzero imped-

ances of the output rectifier, transformer secondary and

output capacitor. Because the secondary current only

flows during the off portion of the duty cycle, the effective

output impedance equals the “DC” lumped secondary

impedance times the inverse of the off duty cycle. If the

output load current remains relatively constant, or, in less

critical applications, the error may be judged acceptable

and the RFB value adjusted for nominal expected error. In

more demanding applications, output impedance error

may be minimized by the use of the load compensation

function (see Load Compensation).

VIN Sense Error

The LT1425 determines the size of the flyback pulse by

comparing the VSW signal to VIN, through RFB. This

comparison is not perfect, in the sense that an offset exists

between the sensing mechanism and the actual VIN. This

is expressed in the data sheet as VIN sense error. This error

is fixed in absolute millivolt terms relative to VOUT (with the

exception that it is reflected to VOUT by any nonunity

secondary-to-primary turns ratio).

MINIMUM LOAD CONSIDERATIONS

The LT1425 generally provides better low load perfor-

mance than previous generation switcher/controllers

utilizing indirect output voltage sensing techniques.

Specifically, it contains circuitry to detect flyback pulse

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