LT1425 查看數據表（PDF） - Linear Technology
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LT1425 Datasheet PDF : 20 Pages
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LT1425
U
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 =
VOUT
(VIN)(Eff)
IOUT
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)
(RSENSE)(G)
ROCOMP
RFB
Expressing the product of RSENSE and G as the data sheet
value of ∆VRCCOMP/∆ISW,
) ) ROUT
=
K1
∆VRCCOMP
∆ISW
RFB
ROCOMP
and,
) ) ROCOMP
=
K1
∆VRCCOMP
∆ISW
RFB
ROUT
where,
K1 = Dimensionless variable related to VIN,
VOUT and efficiency as above
) ∆VRCCOMP
∆ISW
= Data sheet value for RCCOMP pin
action vs switch current
RFB = External “feedback” resistor value
ROUT = Uncompensated output impedance
) ) ∆VOUT
∆IOUT
= K1
∆VRCCOMP
∆ISW
RFB
ROCOMP
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.
10
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