Datasheet LT3512 (Analog Devices) - 9

LT3512
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 3.0W at 72V but of a higher switch voltage. lowers to 2.5W at 36V. The graphs in Figures 1-4 show the typical maximum The equations below calculate output power: output power possible for the output voltages 3.3V, 5V, Power = η • V 12V and 24V. The maximum power output curve is the IN • D • IPEAK • 0.5 calculated output power if the switch voltage is 100V Efficiency = η = ~83% during the off-time. 50V of margin is left for leakage volt- ( ) age spike. To achieve this power level at a given input, a V •N Duty cycle = D = OUT + VF PS winding ratio value must be calculated to stress the switch ( ) VOUT + VF •NPS + VIN to 100V, resulting in some odd ratio values. The following curves are examples of common winding ratio values Peak switch current = IPEAK = 0.44A and the amount of output power at given input voltages. 5.0 5.0 N = 5 N = 15 N = 12 N = 4 N = N 4.0 N = 10 PS(MAX) N = N 4.0 PS(MAX) N = 3 3.0 N = 8 3.0 N = 2 N = 6 2.0 N = 4 2.0 N = 1 OUTPUT POWER (W) OUTPUT POWER (W) 1.0 N = 2 1.0 0 0 0 20 40 60 80 100 0 20 40 60 80 100 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3512 F01 3512 F03
Figure 1. Output Power for 3.3V Output Figure 3. Output Power for 12V Output
5.0 5.0 N = 8 N = NPS(MAX) N = 7 N = N N = 2 4.0 PS(MAX) N = 6 4.0 N = 5 3.0 N = 4 3.0 N = 1 N = 3 2.0 N = 2 2.0 OUTPUT POWER (W) OUTPUT POWER (W) 1.0 N = 1 1.0 0 0 0 20 40 60 80 100 0 20 40 60 80 100 INPUT VOLTAGE (V) INPUT VOLTAGE (V) 3512 F02 3512 F04
Figure 2. Output Power for 5V Output Figure 4. Output Power for 24V Output
3512fb 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Revision History Typical Application Related Parts