Datasheet

LT3512
16
3512fb
applicaTions inForMaTion
Step 1: Select the transformer turns ratio.
N
PS
<
V
SW(MAX)
– V
IN(MAX)
– V
LEAKAGE
V
OUT
+ V
F
V
SW(MAX)
= Max rating of internal switch = 150V
V
LEAKAGE
= Margin for transformer leakage spike = 40V
V
F
= Forward voltage of output diode = assume approxi-
mately ~ 0.5V
Example:
N
PS
<
150V – 72V – 40V
15V +0.5V
N
PS
< 2.45
N
PS
= 2
The choice of turns ratio is critical in determining output
power as shown earlier in the Output Power section. At
this point, a third winding can be added to the transformer
to drive the BIAS pin of the LT3512 for higher efficiencies.
Choose a turns ratio that sets the third winding voltage
to regulate between 3.3V and 6V for maximum efficiency.
Choose a third winding ratio to drive BIAS winding with
5V. (Optional)
Example:
N
THIRD
N
S
=
V
THIRD
V
OUT
=
5V
15V
= 0.33
The turns ratio of the transformer chosen is as follows
N
PRIMARY
: N
SECONDARY
: N
THIRD
= 2:1:0.33.
Step 2: Calculate maximum power output at minimum
V
IN
.
P
OUT(VIN(MIN))
= η • V
IN(MIN)
• I
IN
= η • V
IN(MIN)
• D •
I
PEAK
• 0.5
D=
V
OUT
+ V
F
( )
• N
PS
V
OUT
+ V
F
( )
• N
PS
+ V
IN(MIN)
η = Efficiency = ~83%
I
PEAK
= Peak switch current = 0.44A
Example:
D = 0.46
P
OUT(VIN(MIN))
= 3W
I
OUT(VIN(MIN))
= P
OUT(VIN(MIN))
/V
OUT
= 0.2A
The chosen turns ratio satisfies the output current re-
quirement of 200mA. If the output current was too low,
the minimum input voltage could be adjusted higher. The
turns ratio in this example is set to its highest ratio given
switch voltage requirements and margin for leakage in-
ductance voltage spike.
Step 3: Determine primary inductance, switching
frequency and saturation current.
Primary inductance for the transformer must be set above
a minimum value to satisfy the minimum off and on time
requirements.
L
PRI
≥
t
OFF(MIN)
• N
PS
• V
OUT
+ V
F
( )
I
PEAK(MIN)
t
OFF(MIN)
= 400ns
I
PEAK(MIN)
= 100mA
L
PRI
≥
t
ON(MIN)
• V
IN(MAX)
I
PEAK(MIN)
t
ON(MIN)
= 100ns
I
PEAK(MIN)
= 100mA
Example:
L
PRI
≥
400ns • 2 • 15 + 0.5
( )
0.1
L
PRI
≥ 124µH
L
PRI
≥
100ns • 72
0.1
L
PRI
≥ 72µH