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UC3656G-AF5-R Folha de dados(PDF) 6 Page - Unisonic Technologies |
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UC3656G-AF5-R Folha de dados(HTML) 6 Page - Unisonic Technologies |
6 / 7 page UC3656 Preliminary CMOS IC UNISONICTECHNOLOGIESCO.,LTD 6 of 7 www.unisonic.com.tw QW-R502-469.A APPLICATION INFORMATION Capacitor Selection Proper selection of capacitors using the UTC UC3656 plays a critical role for system performance. The source current of the top MOSFET is square wave of duty cycle in a continuous mode. The input capacitor is generally used to provide a low impedance loop for the edges of pulsed current drawn by the UTC UC3656. A load step at the output can induce ringing at the input VIN. This ringing can couple to the output and be mistaken as loop instability. Increasing the capacitance of the input capacitor can great improve the oscillation. A typical value is 4.7~ 22μF ceramic (X5R or X7R), POSCAP or Aluminum Polymer. With these capacitors, better high frequency bypassing is achieved; resistive losses are thus reduced by their low ESR for higher efficiency. Moreover, the input capacitor RMS current changes with the input voltage and the output voltage. The maximum RMS current in the input capacitor can be expressed in the following formula: ) V V ( V V I I IN O IN O OMAX RMS − = 1 The output capacitor depends upon the suitable ripple voltage. Low ripple voltage corresponds to lower effective series resistance (ESR). The output ripple voltage can be calculated by: ) fC ESR ( I V OUT L OUT 8 1 + Δ ≅ Δ The formula as follows shows the output capacitor RMS ripple current: IN OUT IN OUT RMS V f L ) V V ( V I × × − × = 3 2 1 For the UTC UC3656, X5R or X7R dielectric formulations have the priority in choosing the input and output ceramic capacitors. This is because these dielectrics provide the best temperature and voltage characteristics of all the ceramics for a given value and size. Inductor Selection Selecting an optimum inductor is dependent upon the expected ripple current and large value inductors lower ripple current and small value inductors result in higher ripple current. In addition, the losses associated with the DCR and its effect on the total converter efficiency should be taken consideration. The inductor is also chosen to limit the ripple current to some pre-fixed value, approx. 20 ~ 40% (TYP.) of the full load current at the maximum input voltage. The inductance value can be calculated in the following equation: OUT(MAX) L I 0.4 ~ . I × = Δ 2 0 ) V V ( I f V L IN OUT L OUT − Δ × = 1 Adjustable Output Voltage A resistive divider adjusts the output voltage of the UTC UC3656, which divides a 0.6V reference voltage at FB pin. The output voltage can be obtained by the equation as follows: ) 1 ( 6 . 0 2 1 R R V V OUT + × = |
Nº de peça semelhante - UC3656G-AF5-R |
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Descrição semelhante - UC3656G-AF5-R |
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