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UC3750 датащи(PDF) 6 Page - Unisonic Technologies |
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UC3750 датащи(HTML) 6 Page - Unisonic Technologies |
6 / 8 page UC3750 Preliminary CMOS IC UNISONICTECHNOLOGIESCO.,LTD 6 of 8 www.unisonic.com.tw QW-R502-360.b DESCRIPTION INFORMATION(Cont.) 8. Inductor Value Calculation Selecting the proper inductance must take an inductor’s physical size, transient respond and power conversion requirements into consideration. The fact is that lower inductor value can make the cost, PC board space smaller and can provide faster transient response while resulting in higher ripple current and core losses. The loading current, IOUT = 0.5A and the inductor ripple current, IL−RIPPLE(P−P) is designed to be less than 40% of the load current, i.e. 0.5A x 40% = 0.2A. The relationship between the inductor value and inductor ripple current is as followed: ) P - P ( RIPPLE - I ) V - I × R - V ( × T = L L OUT OUT ) ON ( DS IN ON Where RDS (ON) is the ON resistance of the external P-channel MOSFET. 9. P-Channel Power MOSFET Selection The operation of UTC UC3750 must be used with an external P-Channel power MOSFET. The main keys for the power MOSFET are the gate threshold, VGS, the “ON” resistance, RDS (ON) and its total gate charge, QT. Low input voltage operation need a low gate threshold device that can work down to the minimum input voltage level. RDS (ON) determines the conduction losses for each switching cycle, the lower the ON resistance, the higher and the efficiency can be achieved. A power MOSFET with lower gate charge can give lower switching losses but the fast transient can cause unwanted EMI to the system. Compromise is required during the design stage. 10. Flywheel Diode Selection The flywheel diode is turned on and carries load current during the off time. The average diode current depends on the P-Channel switch duty cycle. At high input voltages, the diode conducts most of the time. In case of VIN approaches VOUT, the diode conducts only a small fraction of the cycle. While the output terminals are shorted, the diode will subject to its highest stress. Under this condition, the diode must be able to safely handle the peak current circulating in the loop. So, it is important to select a flywheel diode that can meet the diode peak current and average power dissipation requirements. Under normal conditions, the average current conducted by the flywheel diode is given by: OUT F IN OUT IN D I × V + V V - V = I Where ID is the average diode current and VF is the forward diode voltage drop. A fast switching diode must also be used to optimize efficiency. Schottky diodes are ideal for low forward drop and fast switching times. 11. Input and Output Capacitor Selection (CIN and COUT) In continuous mode operation, the source current of the P-Channel MOSFET is a square wave of duty cycle (VOUT +VF)/VIN. We must select a low ESR input capacitor that can support the maximum RMS input current to prevent large input voltage transients. The maximum RMS input current, can be estimated by the equation below: IN 2 1 OUT IN OUT OUT ) MAX ( RMS V ) V - V ( V × I I ≈ IRMS (MAX) has a maximum value at VIN = 2VOUT, where IRMS (MAX) = IOUT/2. As a general practice, this simple worst-case condition is used for design. Selecting of the output capacitor, the required effective series resistance (ESR) of the capacitor decide the COUT and the capacitance will be adequate for filtering unless the ESR requirement is met. The output voltage ripple, VRIPPLE is approximated by: ) C F 4 1 (ESR+ × R - I V OUT OSC P) - (P IPPLE L RIPPLE ≈ Where FOSC is the switching frequency and ESR is the effective series resistance of the output capacitor. We can see from equation 4 that the output voltage ripple contributed by two parts. For a better performance, Low ESR tantalum capacitors should be used. The major contributor is the capacitor ESR for most. Ordinary aluminum-electrolytic capacitors have high ESR and should be avoided while higher quality Low ESR aluminum-electrolytic capacitors are acceptable and relatively inexpensive. Surface-mount tantalum capacitors are better and provide neat and compact solution for space sensitive applications. |
Аналогичный номер детали - UC3750 |
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Аналогичное описание - UC3750 |
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