Datasheet

LT3512
9
3512fb
similar to a nonisolated buck-boost. The duty cycle will
affect the input and output currents, making it hard to
predict output power. In addition, the winding ratio can
be changed to multiply the output current at the expense
of a higher switch voltage.
The graphs in Figures 1-4 show the typical maximum
output power possible for the output voltages 3.3V, 5V,
12V and 24V. The maximum power output curve is the
calculated output power if the switch voltage is 100V
during the off-time. 50V of margin is left for leakage volt-
age spike. To achieve this power level at a given input, a
winding ratio value must be calculated to stress the switch
to 100V, resulting in some odd ratio values. The following
curves are examples of common winding ratio values
and the amount of output power at given input voltages.
applicaTions inForMaTion
One design example would be a 5V output converter with
a minimum input voltage of 36V and a maximum input
voltage of 72V. A four-to-one winding ratio fits this design
example perfectly and outputs close to 3.0W at 72V but
lowers to 2.5W at 36V.
The equations below calculate output power:
Power = η • V
IN
• D • I
PEAK
• 0.5
Efficiency = η = ~83%
Duty cycle = D =
V
OUT
+ V
F
( )
• N
PS
V
OUT
+ V
F
( )
• N
PS
+ V
IN
Peak switch current = I
PEAK
= 0.44A
INPUT VOLTAGE (V)
0
0
OUTPUT POWER (W)
1.0
2.0
3.0
4.0
5.0
20 40 60 80
3512 F01
100
N = 12N = 15
N = 10
N = 8
N = 6
N = 4
N = 2
N = N
PS(MAX)
Figure 1. Output Power for 3.3V Output
INPUT VOLTAGE (V)
0
0
OUTPUT POWER (W)
1.0
2.0
3.0
4.0
5.0
20 40 60 80
3512 F02
100
N = 4
N = 5
N = 6
N = 7
N = 8
N = 3
N = 2
N = 1
N = N
PS(MAX)
Figure 2. Output Power for 5V Output
Figure 3. Output Power for 12V Output
Figure 4. Output Power for 24V Output
INPUT VOLTAGE (V)
0
0
OUTPUT POWER (W)
1.0
2.0
3.0
5.0
4.0
20 40 60 80
3512 F03
100
N = 4
N = 5
N = 3
N = 2
N = N
PS(MAX)
N = 1
INPUT VOLTAGE (V)
0
0
OUTPUT POWER (W)
1.0
2.0
3.0
4.0
5.0
20 40 60 80
3512 F04
100
N = 2
N = 1
N = N
PS(MAX)