LTC4258 datasheet(25/42 Pages) LINER

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LTC4278

4278fc

Assuming relatively fixed power supply efficiency, Eff,

power balance gives:

POUT = Eff • PIN

VOUT • IOUT = Eff • VIN • IIN

Average primary-side current is expressed in terms of

output current as follows:

IIN =K1•IOUT

where:

K1=

VOUT

VIN •Eff

So, the effective change in VOUT target is:

ΔVOUT =K1•

RSENSE

RCMP

•R1•NSF • ΔIOUT

thus:

ΔVOUT

ΔIOUT

=K1•

RSENSE

RCMP

•R1•NSF

where:

K1 = dimensionless variable related to VIN, VOUT and

efficiency, as previously explained

RSENSE = external sense resistor

Nominal output impedance cancellation is obtained by

equating this expression with RS(OUT):

K1•

RSENSE

RCMP

•R1•NSF =

ESR

+RDS(ON)

1– DC

Solving for RCMP gives:

RCMP =K1•

RSENSE • 1–DC

(

)

ESR

+RDS(ON)

•R1•NSF

Thepracticalaspectsofapplyingthisequationtodetermine

an appropriate value for the RCMP resistor are discussed

subsequently in the Applications Information section.

Transformer Design

Transformer design/specification is the most critical part

of a successful application of the LTC4278. The following

sections provide basic information about designing the

transformer and potential tradeoffs. If you need help, the

LTC Applications group is available to assist in the choice

and/or design of the transformer.

Turns Ratios

The design of the transformer starts with determining

duty cycle (DC). DC impacts the current and voltage stress

on the power switches, input and output capacitor RMS

currents and transformer utilization (size vs power). The

ideal turns ratio is:

NIIDEAL=

VOUT

VIN

•

1– DC

Avoid extreme duty cycles, as they generally increase cur-

rent stresses. A reasonable target for duty cycle is 50%

at nominal input voltage.

For instance, if we wanted a 48V to 5V converter at 50%

DC then:

NIIDEAL=

•

1– 0.5

0.5

9.6

In general, better performance is obtained with a lower

turns ratio. A DC of 45.5% yields a 1:8 ratio.

APPLICATIONS INFORMATION

•

RCMPF

50k

VIN

VFLBK

R2 LOAD

COMP I

VFB

Q1 Q2

RCMP

CCMP

RSENSE

SENSE+

4278 F11

–

Figure 11. Load Compensation Diagram

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