10
LT1425
OPERATIO
N
U
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
R
OCOMP
resistor and the resulting current is then sub-
tracted from the R
FB
node. As output loading increases,
average switch current increases to maintain rough output
voltage regulation. This causes an increase in R
OCOMP
resistor current subtracted from the R
FB
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)
(V
OUT
)(I
OUT
) = (Eff)(V
IN
)(I
IN
)
Average primary side current may be expressed in terms
of output current as follows:
I
IN
= I
OUT
V
OUT
(V
IN
)(Eff)
)
)
Combining the efficiency and voltage terms in a single
variable,
I
IN
= K1(I
OUT
)
where,
K1
=
V
OUT
(V
IN
)(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 R
OCOMP
resistor to form a current that is
subtracted from the R
FB
node. So the effective change in
V
OUT
target is:
∆
V
OUT
= K1(
∆
I
OUT
) R
FB
(R
SENSE
)(G)
R
OCOMP
)
)
Expressing the product of R
SENSE
and G as the data sheet
value of
∆
V
RCCOMP
/
∆
I
SW
,
∆
V
RCCOMP
∆
I
SW
)
)
∆
V
RCCOMP
∆
I
SW
)
)
R
FB
R
OCOMP
)
)
R
OUT
= K1
and,
R
FB
R
OUT
)
)
R
OCOMP
= K1
∆
V
RCCOMP
∆
I
SW
)
)
= Data sheet value for R
CCOMP
pin
action vs switch current
where,
K1 = Dimensionless variable related to V
IN
,
V
OUT
and efficiency as above
R
FB
= External “feedback” resistor value
R
OUT
= Uncompensated output impedance
∆
V
RCCOMP
∆
I
SW
∆
V
OUT
∆
I
OUT
)
)
R
FB
R
OCOMP
)
)
= K1
Nominal output impedance cancellation is obtained by
equating this expression with R
OUT
. The practical aspects
of applying this equation to determine an appropriate
value for the R
OCOMP
resistor are found in the Applications
Information section.
