Datasheet

LT8302
18
Rev E
For more information www.analog.com
APPLICATIONS INFORMATION
Design Example
Use the following design example as a guide to designing
applications for the LT8302. The design example involves
designing a 5V output with a 1.5A load current and an
input range from 8V to 32V.
V
IN(MIN)
= 8V, V
IN(NOM)
= 12V, V
IN(MAX)
= 32V,
V
OUT
= 5V, I
OUT
= 1.5A
Step 1: Select the transformer turns ratio.
N
PS
<
65V – V
IN(MAX)
– V
LEAKAGE
V
OUT
+ V
F
V
LEAKAGE
= Margin for transformer leakage spike = 15V
V
F
= Output diode forward voltage = ~0.3V
Example:
N
PS
<
65V – 32V – 15V
5V + 0.3V
= 3.4
The choice of transformer turns ratio is critical in deter-
mining output current capability of the converter. Table2
shows the switch voltage stress and output current capa-
bility at different transformer turns ratio.
Table2. Switch Voltage Stress and Output Current Capability vs
Turns Ratio
NPS
V
SW(MAX)
at
V
IN(MAX)
(V)
I
OUT(MAX)
at
V
IN(MIN)
(A) DUTY CYCLE (%)
1:1 37.3 0.92 14-40
2:1 42.6 1.31 25-57
3:1 47.9 1.53 33-67
Clearly, only N
PS
= 3 can meet the 1.5A output current
requirement, so N
PS
= 3 is chosen as the turns ratio in
this example.
Step 2: Determine the primary inductance.
Primary inductance for the transformer must be set above
a minimum value to satisfy the minimum switch-off and
switch-on time requirements:
L
PRI
≥
t
OFF(MIN)
•N
PS
• V
OUT
+ V
F
( )
I
SW(MIN)
L
PRI
≥
t
ON(MIN)
• V
IN(MAX)
I
SW(MIN)
t
OFF(MIN)
= 350ns
t
ON(MIN)
= 160ns
I
SW(MIN)
= 0.87A
Example:
L
PRI
≥
350ns • 3 • 5V + 0.3V
( )
0.87A
= 6.4µH
L
PRI
≥
160ns • 32V
0.87A
= 5.9µH
Most transformers specify primary inductance with a tol-
erance of ±20%. With other component tolerance consid-
ered, choose a transformer with its primary inductance
40% to 60% larger than the minimum values calculated
above. L
PRI
= 9µH is then chosen in this example.
Once the primary inductance has been determined, the
maximum load switching frequency can be calculated as:
f
SW
=
1
t
ON
+ t
OFF
=
1
L
PRI
•I
SW
V
IN
+
L
PRI
•I
SW
N
PS
• V
OUT
+ V
F
( )
I
SW
=
V
OUT
•I
OUT
• 2
η • V
IN
•D
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