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NCP3418B Folha de dados(PDF) 6 Page - ON Semiconductor |
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NCP3418B Folha de dados(HTML) 6 Page - ON Semiconductor |
6 / 8 page NCP3418B http://onsemi.com 6 Safety Timer and Overlap Protection Circuit It is very important that MOSFETs in a synchronous buck regulator do not both conduct at the same time. Excessive shoot−through or cross conduction can damage the MOSFETs, and even a small amount of cross conduction will cause a decrease in the power conversion efficiency. The NCP3418B prevents cross conduction by monitoring the status of the external mosfets and applying the appropriate amount of “dead−time” or the time between the turn off of one MOSFET and the turn on of the other MOSFET. When the PWM input pin goes high, DRVL will go low after a propagation delay (tpdlDRVL). The time it takes for the low−side MOSFET to turn off (tfDRVL) is dependent on the total charge on the low−side MOSFET gate. The NCP3418B monitors the gate voltage of both MOSFETs and the switchnode voltage to determine the conduction status of the MOSFETs. Once the low−side MOSFET is turned off an internal timer will delay (tpdhDRVH) the turn on of the high−side MOSFET Likewise, when the PWM input pin goes low, DRVH will go low after the propagation delay (tpdDRVH). The time to turn off the high−side MOSFET (tfDRVH) is dependent on the total gate charge of the high−side MOSFET. A timer will be triggered once the high−side mosfet has stopped conducting, to delay (tpdhDRVL) the turn on of the low−side MOSFET Power Supply Decoupling The NCP3418B can source and sink relatively large currents to the gate pins of the external MOSFETs. In order to maintain a constant and stable supply voltage (Vcc) a low ESR capacitor should be placed near the power and ground pins. A 1 mF to 4.7 mF multi layer ceramic capacitor (MLCC) is usually sufficient. Input Pins The PWM input and the Output Disable pins of the NCP3418B have internal protection for Electro Static Discharge (ESD), but in normal operation they present a relatively high input impedance. If the PWM controller does not have internal pull−down resistors, they should be added externally to ensure that the driver outputs do not go high before the controller has reached its under voltage lockout threshold. The NCP5381 controller does include a passive internal pull−down resistor on the drive−on output pin. Bootstrap Circuit The bootstrap circuit uses a charge storage capacitor (CBST) and the internal (or an external) diode. Selection of these components can be done after the high−side MOSFET has been chosen. The bootstrap capacitor must have a voltage rating that is able to withstand twice the maximum supply voltage. A minimum 50 V rating is recommended. The capacitance is determined using the following equation: CBST + QGATE DVBST where QGATE is the total gate charge of the high−side MOSFET, and DVBST is the voltage droop allowed on the high−side MOSFET drive. For example, a NTD60N03 has a total gate charge of about 30 nC. For an allowed droop of 300 mV, the required bootstrap capacitance is 100 nF. A good quality ceramic capacitor should be used. The bootstrap diode must be rated to withstand the maximum supply voltage plus any peak ringing voltages that may be present on SW. The average forward current can be estimated by: IF(AVG) + QGATE fMAX where fMAX is the maximum switching frequency of the controller. The peak surge current rating should be checked in−circuit, since this is dependent on the source impedance of the 12 V supply and the ESR of CBST. NCP3418 4 3 2 5 6 7 8 1 Vcc OD IN DRVL PGND SW DRVH BST Vout 12 V Output Enable 12 V PWM in Figure 4. NCP3418 Example Circuit |
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Descrição semelhante - NCP3418B |
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