LT1425CS datasheet(10/20 Pages) LINER

10 / 20 page

LT1425

OPERATION

may be minimized by the use of the load compensation

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

ROCOMP resistor and the resulting current is then sub-

tracted from the RFB node. As output loading increases,

average switch current increases to maintain rough output

voltage regulation. This causes an increase in ROCOMP

resistor current subtracted from the RFB node, through

which feedback loop action causes 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:

IIN =

IOUT

VOUT

(VIN)(Eff)

)

Combining the efficiency and voltage terms in a single

variable,

IIN = K1(IOUT)

where,

K1 =

VOUT

(VIN)(Eff)

)

Switch current is converted to voltage by a sense resistor

and amplified by the current sense amplifier with associ-

ated gain G. This voltage is then impressed across the

external ROCOMP resistor to form a current that is

subtracted from the RFB node. So the effective change in

VOUT target is:

∆VOUT = K1(∆IOUT)

RFB

(RSENSE)(G)

ROCOMP

)

Expressing the product of RSENSE and G as the data sheet

value of

∆VRCCOMP/∆ISW,

∆VRCCOMP

∆ISW

)

∆VRCCOMP

∆ISW

)

RFB

ROCOMP

)

ROUT = K1

and,

RFB

ROUT

)

ROCOMP = K1

∆VRCCOMP

∆ISW

)

= Data sheet value for RCCOMP pin

action vs switch current

where,

K1 = Dimensionless variable related to VIN,

VOUT and efficiency as above

RFB = External “feedback” resistor value

ROUT = Uncompensated output impedance

∆VRCCOMP

∆ISW

∆VOUT

∆IOUT

)

RFB

ROCOMP

)

= K1

Nominal output impedance cancellation is obtained by

equating this expression with ROUT. The practical aspects

of applying this equation to determine an appropriate

value for the ROCOMP resistor are found in the Applications

Information section.

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