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TC115301EMTTR датащи(PDF) 6 Page - Microchip Technology |
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TC115301EMTTR датащи(HTML) 6 Page - Microchip Technology |
6 / 14 page TC115 DS21361C-page 6 2003 Microchip Technology Inc. 5.0 APPLICATIONS 5.1 Input Bypass Capacitors Using an input bypass capacitor reduces peak current transients drawn from the input supply and reduces the switching noise generated by the regulator. The source impedance of the input supply determines the size of the capacitor that should be used. FIGURE 5-1: TC115 Typical Application. 5.2 Inductor Selection Selecting the proper inductor value is a trade-off between physical size and power conversion require- ments. Lower value inductors cost less, but result in higher ripple current and core losses. They are also more prone to saturate since the coil current ramps to a higher value. Larger inductor values reduce both ripple current and core losses, but are larger in physical size and tend to increase the start-up time slightly. Practical inductor values, therefore, range from 50 µH to 300 µH. Inductors with a ferrite core (or equivalent) are recommended. For highest efficiency, use an inductor with a series resistance less than 0.1 Ω). The inductor value directly affects the output ripple voltage. Equation 5-3 is derived as shown below, and can be used to calculate an inductor value, given the required output ripple voltage (VRIPPLE) and output capacitor series resistance: EQUATION 5-1: Expressing di in terms of switch ON resistance and time: EQUATION 5-2: Solving for L: EQUATION 5-3: Care must be taken to ensure the inductor can handle peak switching currents, which can be several times load currents. Exceeding rated peak current will result in core saturation and loss of inductance. The inductor should be selected to withstand currents greater than IPK (Equation 5-10) without saturating. Calculating the peak inductor current is straightfor- ward. Inductor current consists of an AC (sawtooth) current centered on an average DC current (i.e., input current). Equation 5-6 calculates the average DC cur- rent. Note that minimum input voltage and maximum load current values should be used: EQUATION 5-4: C1 L1 5 4 TC115 1 3 2 SHDN PS NC C2 D1 GND LX VOUT VIN OFF ON (Tie to VIN or VOUT if not used) + + Where: ESR: the equivalent series resistance of the output filter capacitor; VRIPPLE is in volts. di: represents the peak to peak ripple current in the inductor. VRIPPLE ESR di () ≈ VRIPPLE VIN VSW – ()tON [] L -------------------------------------------- ≈ Where: VSW = voltage drop across the switch. TON = the amount of time the switch is ON. L VIN VSW – ()tON [] VRIPPLE -------------------------------------------- ≈ Input Power Output Power Efficiency --------------------------------- = |
Аналогичный номер детали - TC115301EMTTR |
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Аналогичное описание - TC115301EMTTR |
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