Datasheet

LT8302
12
Rev E
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APPLICATIONS INFORMATION
To cancel the output diode temperature coefficient, the
following two equations should be satisfied:
V
OUT
= V
REF
•
R
FB
R
REF
•
1
N
PS
– V
F
TO
( )
V
TC
/ T
( )
•
R
FB
R
TC
•
1
N
PS
= – V
F
/ T
( )
Selecting Actual R
REF
, R
FB
, R
TC
Resistor Values
The LT8302 uses a unique sampling scheme to regulate
the isolated output voltage. Due to the sampling nature,
the scheme contains repeatable delays and error sources,
which will affect the output voltage and force a re-eval-
uation of the R
FB
and R
TC
resistor values. Therefore, a
simple 2-step sequential process is recommended for
selecting resistor values.
Rearrangement of the expression for V
OUT
in the previous
sections yields the starting value for R
FB
:
R
FB
=
R
REF
•N
PS
• V
OUT
+ V
F
TO
( )
( )
V
REF
V
OUT
= Output voltage
V
F
(TO) = Output diode forward voltage at 25°C = ~0.3V
N
PS
= Transformer effective primary-to-secondary
turns ratio
The equation shows that the R
FB
resistor value is indepen-
dent of the R
TC
resistor value. Any R
TC
resistor connected
between the TC and R
REF
pins has no effect on the output
voltage setting at 25°C because the TC pin voltage is equal
to the R
REF
regulation voltage at 25°C.
The R
REF
resistor value should be approximately 10k
because the LT8302 is trimmed and specified using this
value. If the R
REF
resistor value varies considerably from
10k, additional errors will result. However, a variation in
R
REF
up to 10% is acceptable. This yields a bit of freedom
in selecting standard 1% resistor values to yield nominal
R
FB
/R
REF
ratios.
First, build and power up the application with the starting
R
REF
, R
FB
values (no R
TC
resistor yet) and other compo-
nents connected, and measure the regulated output volt-
age, V
OUT(MEAS)
. The new R
FB
value can be adjusted to:
R
FB(NEW)
=
V
OUT
V
OUT(MEAS)
•R
FB
Second, with a new R
FB
resistor value selected, the output
diode temperature coefficient in the application can be
tested to determine the R
TC
value. Still without the R
TC
resistor, the V
OUT
should be measured over temperature
at a desired target output load. It is very important for
this evaluation that uniform temperature be applied to
both the output diode and the LT8302. If freeze spray or
a heat gun is used, there can be a significant mismatch
in temperature between the two devices that causes sig
-
nificant error. Attempting to extrapolate the data from a
diode data sheet is another option if there is no method
to apply uniform heating or cooling such as an oven. With
at least two data points spreading across the operating
temperature range, the output diode temperature coef-
ficient can be determined by:
– δV
F
/δT
( )
=
V
OUT
T1
( )
– V
OUT
T2
( )
T1– T2
Using the measured output diode temperature coefficient,
an exact R
TC
value can be selected with the following
equation:
R
TC
=
δV
TC
/δT
( )
– δV
F
/δT
( )
•
R
FB
N
PS
⎛
⎝
⎜
⎞
⎠
⎟
Once the R
REF
, R
FB
, and R
TC
values are selected, the regu-
lation accuracy from board to board for a given applica-
tion will be very consistent, typically under ±5% when
including device variation of all the components in the
system (assuming resistor tolerances and transformer
windings matching within ±1%). However, if the trans-
former or the output diode is changed, or the layout is
dramatically altered, there may be some change in V
OUT
.
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