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LT3511

3511fa

applicaTions inForMaTion

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.

ThegraphsinFigures1-4showthetypicalmaximumoutput

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 voltage 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.

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 1.6W at 72V but

lowers to 1W at 36V.

The equations below calculate output power:

Power = η • VIN • D • IPEAK • 0.5

Efficiency = η = ~85%

Duty Cycle = D =

VOUT + VF

(

) •NPS

VOUT + VF

(

) •NPS + VIN

Peak switch current = IPEAK = 0.26AV

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

N=15

3511 F01

100

N=12

N=10

N=8

N=6

N=4

N=2

N = NPS(MAX)

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3511 F02

100

N=4

N=5

N=6

N=7

N=8

N=3

N=2

N=1

N = NPS(MAX)

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.0

3511 F04

100

N=2

N=1

N = NPS(MAX)

Figure 1. Output Power for 3.3V Output

Figure 2. Output Power for 5V Output

Figure 3. Output Power for 12V Output

Figure 4. Output Power for 24V Output

INPUT VOLTAGE (V)

0.5

1.0

1.5

2.0

2.5

3.5

3.0

3511 F03

100

N=4

N=5

N=3

N=2

N=1

N = NPS(MAX)

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