Linear Technology LT8302, Operation, Page: 20 / 26

LT8302
20
8302fb
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applicaTions inForMaTion
Step 6: Select the R
REF
and R
FB
resistors.
Use the following equation to calculate the starting values
for R
REF
and R
FB
:
R
FB
=
R
REF
•N
PS
• V
OUT
+
V
F
TO
( )
( )
V
REF
R
REF
= 10k
Example:
R
FB
=
10k • 3 • 5V + 0.3V
( )
1.00V
= 159k
For 1% standard values, a 158k resistor is chosen.
Step 7: Adjust R
FB
resistor based on output voltage.
Build and power up the application with application com-
ponents and measure the regulated output voltage. Adjust
R
FB
resistor based on the measured output voltage:
R
FB(NEW)
=
V
OUT
V
OUT(MEASURED)
•R
FB
Example:
R
FB
=
5V
5.14V
• 158k = 154k
Step 8: Select R
TC
resistor based on output voltage
temperature variation.
Measure output voltage in a controlled temperature envi-
ronment like an oven to determine the output temperature
coefficient. Measure output voltage at a consistent load
current and input voltage, across the operating tempera-
ture range.
Calculate the temperature coefficient of V
F
:
– δ V
F
/ δ T
( )
=
V
OUT
T1
( )
– V
OUT
T2
( )
T1– T2
R
TC
=
3.35mV/ ° C
– δ V
F
/ δ T
( )
•
R
FB
N
PS
⎛
⎝⎜ ⎞
⎠⎟
Example:
–
δ
V
F
/
δ
T
( )
=
5.189V – 5.041V
100 ° C – 0 ° C
( )
=
1.48mV /
°
C
R
TC
=
3.35mV/
°
C
1.48mV/
°
C
•
154
3
⎛
⎝⎜
⎞
⎠⎟ =
115k
Step 9: Select the EN/UVLO resistors.
Determine the amount of hysteresis required and calculate
R1 resistor value:
V
IN(HYS)
= 2.5µA • R1
Example:
Choose 2V of hysteresis, R1 = 806k
Determine the UVLO thresholds and calculate R2 resistor
value:
V
IN(UVLO + )
=
1.228V • R1
+
R2
( )
R2
+
2.5µA •R1
Example:
Set V
IN
UVLO rising threshold to 7.5V:
R2 = 232k
V
IN(UVLO
+
)
= 7.5V
V
IN(UNLO
–
)
= 5.5V
Step 10: Ensure minimum load.
The theoretical minimum load can be approximately
estimated as:
I
LOAD(MIN)
=
9µH • 0.96A
( ) 2
• 12.7kHz
2 • 5V
= 10.5mA
Remember to check the minimum load requirement in
real application. The minimum load occurs at the point
where the output voltage begins to climb up as the con-
verter delivers more energy than what is consumed at
the output. The real minimum load for this application is
about 10mA. In this example, a 500Ω resistor is selected
as the minimum load.