LT1425 Linear Technology, LT1425 Datasheet - Page 11

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

APPLICATIONS
SELECTING R
The expression for V
section can be rearranged to yield the following expres-
sion for R
The unknown parameter , which represents the fraction
of R
sented instead by specified data sheet values as follows:
Allowing the expression for R
Strictly speaking, the above equation defines R
absolute value, but as a ratio of R
is, “What is the proper value for R
R
LT1425 is trimmed and specified using this value of R
If the impedance of R
additional errors will result. However, a variation in R
of several percent or so is perfectly acceptable. This yields
a bit of freedom in selecting standard 1% resistor values
to yield nominal R
SELECTING R
The Operation section previously derived the following
expressions for R
REF
R
(I
R
where,
V
V
(I
I
N
REF
FB
OUT
F
FB
REF
FB
SEC
SP
=
should be approximately 3k. This is because the
= Switching diode forward voltage
current flowing into the R
= R
= R
= Data sheet reference current value
= Effective secondary-to-primary turns ratio
)( )(3k) = V
)(ESR) = Secondary resistive losses
(I
= Desired output voltage
REF
FB
REF
V
REF
:
BG
)(3k)
FB
OCOMP
V
V
AND R
OUT
OUT
OUT
FB
U
/R
, i.e., effective output impedance and
BG
RESISTOR VALUE
I
+ V
+ V
OUT
REF
REF
REF
REF
INFORMATION
F
U
V
F
(3k)N
ratios.
varies considerably from 3k,
+ I
RESISTOR VALUES
developed in the Operation
BG
+ I
FB
SEC
SEC
REF
to be rewritten as:
SP
REF
REF
(ESR)
(ESR)
W
. So the next question
?” The answer is that
node, can be repre-
N
SP
FB
U
not as an
REF
REF
.
R
nal compensation:
While the value for R
determined, it is usually better in practice to employ
empirical methods. This is because several of the required
input variables are difficult to estimate precisely. For
instance, the ESR term above includes that of the trans-
former secondary, but its effective ESR value depends on
high frequency behavior, not simply DC winding resis-
tance. Similarly, K1 appears to be a simple ratio of V
V
mating efficiency is not a simple calculation. The sug-
gested empirical method is as follows:
OUT
OCOMP
Build a prototype of the desired supply using the
eventual secondary components. Temporarily ground
the R
tion. Operate the supply over the expected range of
output current loading while measuring the output
voltage deviation. Approximate this variation as a single
value of R
value for the K1 constant based on V
measured (differential) efficiency. They are then com-
bined with the data sheet typical value for ( V
Verify this result by connecting a resistor of roughly this
value from the R
ground short to R
0.1 F filter capacitor to ground.) Measure the output
impedance with the new compensation in place. Modify
the original R
decrease the effective compensation.
Once the proper load compensation resistor has been
chosen, it may be necessary to adjust the value of the
R
system exhibits some nonlinearity. In particular, the
circuit can shift the reference current by a noticeable
R
R
I
FB
OUT
OCOMP
SW
times (differential) efficiency, but theoretically esti-
resistor. This is because the load compensation
, the external resistor value required for its nomi-
CCOMP
) to yield a value for R
= ESR
= K1
OUT
pin to disable the load compensation func-
(straight line approximation). Calculate a
OCOMP
1 – DC
OCOMP
1
V
OCOMP
RCCOMP
CCOMP
I
SW
value if necessary to increase or
pin to ground. (Disconnect the
may therefore be theoretically
and connect the requisite
OCOMP
R
R
OUT
FB
.
IN
, V
LT1425
OUT
RCCOMP
and the
11
IN
to
/