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PE53829S Datasheet(PDF) 11 Page - ON Semiconductor |
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PE53829S Datasheet(HTML) 11 Page - ON Semiconductor |
11 / 28 page LM2575, NCV2575 http://onsemi.com 11 Procedure (Fixed Output Voltage Version) (continued)In order to simplify the switching regulator design, a step−by−step design procedure and example is provided. Procedure Example 5. Output Capacitor Selection (Cout) A. Since the LM2575 is a forward−mode switching regulator with voltage mode control, its open loop 2−pole−2−zero frequency characteristic has the dominant pole−pair determined by the output capacitor and inductor values. For stable operation and an acceptable ripple voltage, (approximately 1% of the output voltage) a value between 100 mF and 470 mF is recommended. B. Due to the fact that the higher voltage electrolytic capacitors generally have lower ESR (Equivalent Series Resistance) numbers, the output capacitor’s voltage rating should be at least 1.5 times greater than the output voltage. For a 5.0 V regulator, a rating at least 8V is appropriate, and a 10 V or 16 V rating is recommended. 5. Output Capacitor Selection (Cout) A. Cout = 100 mF to 470 mF standard aluminium electrolytic. B. Capacitor voltage rating = 16 V. Procedure (Adjustable Output Version: LM2575−Adj) Procedure Example Given Parameters: Vout = Regulated Output Voltage Vin(max) = Maximum DC Input Voltage ILoad(max) = Maximum Load Current Given Parameters: Vout = 8.0 V Vin(max) = 12 V ILoad(max) = 1.0 A 1. Programming Output Voltage To select the right programming resistor R1 and R2 value (see Figure 14) use the following formula: Resistor R1 can be between 1.0 k and 5.0 kW. (For best temperature coefficient and stability with time, use 1% metal film resistors). Vout + Vref 1 ) R2 R1 R2 + R1 Vout Vref 1 where Vref = 1.23 V 1. Programming Output Voltage (selecting R1 and R2) Select R1 and R2: R2 = 9.91 kW, choose a 9.88 k metal film resistor. R2 + R1 Vout Vref * 1 + 1.8 k 8.0 V 1.23 V * 1 Vout + 1.23 1 ) R2 R1 Select R1 = 1.8 kW 2. Input Capacitor Selection (Cin) To prevent large voltage transients from appearing at the input and for stable operation of the converter, an aluminium or tantalum electrolytic bypass capacitor is needed between the input pin +Vin and ground pin GND This capacitor should be located close to the IC using short leads. This capacitor should have a low ESR (Equivalent Series Resistance) value. For additional information see input capacitor section in the “External Components” section of this data sheet. 2. Input Capacitor Selection (Cin) A 100 mF aluminium electrolytic capacitor located near the input and ground pin provides sufficient bypassing. 3. Catch Diode Selection (D1) A. Since the diode maximum peak current exceeds the regulator maximum load current the catch diode current rating must be at least 1.2 times greater than the maximum load current. For a robust design, the diode should have a current rating equal to the maximum current limit of the LM2575 to be able to withstand a continuous output short. B. The reverse voltage rating of the diode should be at least 1.25 times the maximum input voltage. 3. Catch Diode Selection (D1) A. For this example, a 3.0 A current rating is adequate. B. Use a 20 V 1N5820 or MBR320 Schottky diode or any suggested fast recovery diode in the Table 4. |
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