LT1425 Linear Technology, LT1425 Datasheet - Page 10

• Manufacturer Part Number: LT1425
• Description: Isolated Flyback Switching Regulator
• Manufacturer: Linear Technology

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