LT1725IGN datasheet(11/28 Pages) LINER

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LT1725

1725f

transformer secondary and output capacitor. This has

been represented previously by the expression “ISEC •

ESR.” However, it is generally more useful to convert this

expression to an effective output impedance. Because the

secondary current only flows during the off portion of the

duty cycle, the effective output impedance equals the

lumped secondary impedance times the inverse of the OFF

duty cycle. That is:

ESR

OUT

OFF









where

ROUT = effective supply output impedance

ESR = lumped secondary impedance

DCOFF = OFF duty cycle

Expressing this in terms of the ON duty cycle, remember-

ing DCOFF = 1 – DC,

ESR

OUT =









1–

DC = ON duty cycle

In less critical applications, or if output load current

remains relatively constant, this output impedance error

may be judged acceptable and the external FB resistor

divider adjusted to compensate for nominal expected

error. In more demanding applications, output impedance

error may be minimized by the use of the load compensa-

tion function.

To implement the load compensation function, a voltage is

developed that is proportional to average output switch

current. This voltage is then impressed across the external

ROCMP resistor, and the resulting current acts to increase

the VBG reference used by the flyback error amplifier. As

output loading increases, average switch current increases

to maintain rough output voltage regulation. This causes

an increase in ROCMP resistor current which effects a

corresponding increase in target output voltage.

Assuming a relatively fixed power supply efficiency, Eff,

Power Out = Eff • Power In

VOUT • IOUT = Eff • VIN • IIN

Average primary side current may be expressed in terms

of output current as follows:

OPERATIO

EFF

OUT

= 







•

combining the efficiency and voltage terms in a single

variable:

IIN = K1 • IOUT, where

EFF

OUT

= 







•

Switch current is converted to voltage by the external

sense resistor and averaged/lowpass filtered by R3 and

the external capacitor on RCMPC. This voltage is then

impressed across the external ROCMP resistor by op amp

A1 and transistor Q3. This produces a current at the

collector of Q3 which is then mirrored around and then

subtracted from the FB node. This action effectively in-

creases the voltage required at the top of the R1/R2

feedback divider to achieve equilibrium. So the effective

change in VOUT target is:

∆=∆

()









∆









OUT

SENSE

OCMP

OUT

SENSE

OCMP

•• (

)

• (

)

Nominal output impedance cancellation is obtained by

equating this expression with ROUT:

and

where

OUT

SENSE

OCMP

SENSE

OUT

















• (

)

• (

)

K1 = dimensionless variable related to VIN, VOUT and

efficiency as above

RSENSE = external sense resistor

ROUT = uncompensated output impedance

(R1||R2) = impedance of R1 and R2 in parallel

The practical aspects of applying this equation to deter-

mine an appropriate value for the ROCMP resistor are found

in the Applications Information section.

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