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ADP3418 Arkusz danych(PDF) 10 Page - Analog Devices |
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ADP3418 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 16 page ADP3418 Rev. B | Page 10 of 16 APPLICATION INFORMATION SUPPLY CAPACITOR SELECTION For the supply input (VCC) of the ADP3418, a local bypass capacitor is recommended to reduce the noise and to supply some of the peak currents drawn. Use a 4.7 µF, low ESR capacitor. Multilayer ceramic chip (MLCC) capacitors provide the best combination of low ESR and small size. Keep the ceramic capacitor as close as possible to the ADP3418. BOOTSTRAP CIRCUIT The bootstrap circuit uses a charge storage capacitor (CBST) and a diode, as shown in Figure 1. These components can be selected after the high-side MOSFET has been chosen. The bootstrap capacitor must have a voltage rating that is able to handle twice the maximum supply voltage. A minimum 50 V rating is recommended. The capacitor values are determined using the following equations: GATE GATE BST2 BST1 V Q C C × = + 10 (1) D GATE BST2 BST1 BST1 V VCC V C C C − = + (2) where QGATE is the total gate charge of the high-side MOSFET at VGATE, VGATE is the desired gate drive voltage (usually in the range of 5-10 V, 7 V being typical), and VD is the voltage drop across D1. Rearranging Equations 1 and 2 to solve for CBST1 yields D GATE BST1 V VCC Q C − × = 10 CBST2 can then be found by rearranging Equation 1: 1 10 BST GATE GATE BST2 C V Q C − × = For example, an NTD60N02 has a total gate charge of about 12 nC at VGATE = 7 V. Using VCC = 12 V and VD = 1 V, we find CBST1 = 12 nF and CBST2 = 6.8 nF. Good quality ceramic capacitors should be used. RBST is used for slew-rate limiting to minimize the ringing at the switch node. It also provides peak current limiting through D1. An RBST value of 1.5 Ω to 2.2 Ω is a good choice. The resistor needs to be able to handle at least 250 mW due to the peak currents that flow through it. A small-signal diode can be used for the bootstrap diode due to the ample gate drive voltage supplied by VCC. The bootstrap diode must have a minimum 15 V rating to withstand the maximum supply voltage. The average forward current can be estimated by MAX GATE AVG F f Q I × = ) ( (3) where fMAX is the maximum switching frequency of the controller. The peak surge current rating should be calculated using: BST D PEAK F R V VCC I − = ) ( (4) MOSFET SELECTION When interfacing the ADP3418 to external MOSFETs, there are a few considerations that the designer should be aware of. These will help to make a more robust design that will minimize stresses on both the driver and MOSFETs. These stresses include exceeding the short-time duration voltage ratings on the driver pins as well as the external MOSFET. It is also highly recommended to use the Boot-Snap circuit to improve the interaction of the driver with the characteristics of the MOSFETs. If a simple bootstrap arrangement is used, make sure to then include a proper snubber network on the SW node. High-Side (Control) MOSFETs The high-side MOSFET is usually selected to be high speed to minimize switching losses (see any ADI Flex-mode™ controller datasheet for more details on MOSFET losses). This usually implies a low gate resistance and low input capacitance/charge device. Yet, there is also a significant source lead inductance that can exist (this depends mainly on the MOSFET package; it is best to contact the MOSFET vendor for this information). The ADP3418 DRVH output impedance and the input resistance of the MOSFETs determine the rate of charge delivery to the gate’s internal capacitance, which determines the speed at which the MOSFETs turn on and off. However, due to potentially large currents flowing in the MOSFETs at the on and off times (this current is usually larger at turn off due to ramping up of the output current in the output inductor), the source lead inductance will generate a significant voltage across it when the high-side MOSFETs switch off. This will create a significant drain-source voltage spike across the internal die of the MOSFETs and can lead to catastrophic avalanche. The mechanisms involved in this avalanche condition can be referenced in literature from the MOSFET suppliers. |
Podobny numer części - ADP3418 |
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Podobny opis - ADP3418 |
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