Datasheet LT3511 (Analog Devices) - 9

LT3511
APPLICATIONS INFORMATION
similar to a nonisolated buck-boost. The duty cycle will One design example would be a 5V output converter with affect the input and output currents, making it hard to a minimum input voltage of 36V and a maximum input predict output power. In addition, the winding ratio can voltage of 72V. A four-to-one winding ratio fits this design be changed to multiply the output current at the expense example perfectly and outputs close to 1.6W at 72V but of a higher switch voltage. lowers to 1W at 36V. The graphs in Figures 1-4 show the typical maximum output The equations below calculate output power: power possible for the output voltages 3.3V, 5V, 12V and Power = η • V 24V. The maximum power output curve is the calculated IN • D • IPEAK • 0.5 output power if the switch voltage is 100V during the off- Efficiency = η = ~85% time. 50V of margin is left for leakage voltage spike. To ( + V ) F • N achieve this power level at a given input, a winding ratio Duty Cycle = D = VOUT PS (V + V ) •N + V value must be calculated to stress the switch to 100V, OUT F PS IN resulting in some odd ratio values. The following curves are examples of common winding ratio values and the Peak switch current = IPEAK = 0.26A amount of output power at given input voltages. 3.0 3.5 2.5 3.0 N = 15 N = 12 N = N N = 5 N = 4 PS(MAX) N = 10 2.5 2.0 N = 8 N = NPS(MAX) N = 3 N = 6 2.0 1.5 N = 2 N = 4 1.5 1.0 N = 1 OUTPUT POWER (W) OUTPUT POWER (W) 1.0 N = 2 0.5 0.5 0 0 0 20 40 60 80 100 0 20 40 60 80 100 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3511 F01 3511 F03
Figure 1. Output Power for 3.3V Output Figure 3. Output Power for 12V Output
3.0 3.0 N = 8 2.5 2.5 N = N N = 7 PS(MAX) N = NPS(MAX) N = 6 N = 2 2.0 N = 5 2.0 N = 4 N = 1 1.5 N = 3 1.5 N = 2 1.0 1.0 OUTPUT POWER (W) OUTPUT POWER (W) N = 1 0.5 0.5 0 0 0 20 40 60 80 100 0 20 40 60 80 100 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3511 F02 3511 F04
Figure 2. Output Power for 5V Output Figure 4. Output Power for 24V Output
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