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FDMF6821A Datasheet(PDF) 17 Page - ON Semiconductor |
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FDMF6821A Datasheet(HTML) 17 Page - ON Semiconductor |
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17 / 21 page  © 2012 Fairchild Semiconductor Corporation www.fairchildsemi.com FDMF6821A • Rev. 1.0.0 16 PCB Layout Guidelines Figure 31 and Figure 32 provide an example of a proper layout for the FDMF6821A and critical components. All of the high-current paths, such as VIN, VSWH, VOUT, and GND copper, should be short and wide for low inductance and resistance. This aids in achieving a more stable and evenly distributed current flow, along with enhanced heat radiation and system performance. Recommendations for PCB Designers 1. Input ceramic bypass capacitors must be placed close to the VIN and PGND pins. This helps reduce the high-current power loop inductance and the input current ripple induced by the power MOSFET switching operation. 2. The VSWH copper trace serves two purposes. In addition to being the high-frequency current path from the DrMOS package to the output inductor, it serves as a heat sink for the low-side MOSFET in the DrMOS package. The trace should be short and wide enough to present a low-impedance path for the high-frequency, high-current flow between the DrMOS and inductor. The short and wide trace minimizes electrical losses as well as the DrMOS temperature rise. Note that the VSWH node is a high- voltage and high-frequency switching node with high noise potential. Care should be taken to minimize coupling to adjacent traces. Since this copper trace acts as a heat sink for the lower MOSFET, balance using the largest area possible to improve DrMOS cooling while maintaining acceptable noise emission. 3. An output inductor should be located close to the FDMF6821A to minimize the power loss due to the VSWH copper trace. Care should also be taken so the inductor dissipation does not heat the DrMOS. 4. PowerTrench ® MOSFETs are used in the output stage and are effective at minimizing ringing due to fast switching. In most cases, no VSWH snubber is required. If a snubber is used, it should be placed close to the VSWH and PGND pins. The selected resistor and capacitor need to be the proper size for power dissipation. 5. VCIN, VDRV, and BOOT capacitors should be placed as close as possible to the VCIN-to-CGND, VDRV-to-CGND, and BOOT-to-PHASE pin pairs to ensure clean and stable power. Routing width and length should be considered as well. 6. Include a trace from the PHASE pin to the VSWH pin to improve noise margin. Keep this trace as short as possible. 7. The layout should include the option to insert a small-value series boot resistor between the boot capacitor and BOOT pin. The boot-loop size, including RBOOT and CBOOT, should be as small as possible. The boot resistor may be required when operating above 15 VIN and is effective at controlling the high-side MOSFET turn-on slew rate and VSHW overshoot. RBOOT can improve noise operating margin in synchronous buck designs that may have noise issues due to ground bounce or high positive and negative VSWH ringing. Inserting a boot resistance lowers the DrMOS efficiency. Efficiency versus noise trade-offs must be considered. RBOOT values from 0.5 Ω to 3.0 Ω are typically effective in reducing VSWH overshoot. 8. The VIN and PGND pins handle large current transients with frequency components greater than 100 MHz. If possible, these pins should be connected directly to the VIN and board GND planes. The use of thermal relief traces in series with these pins is discouraged since this adds inductance to the power path. This added inductance in series with either the VIN or PGND pin degrades system noise immunity by increasing positive and negative VSWH ringing. 9. GND pad and PGND pins should be connected to the GND copper plane with multiple vias for stable grounding. Poor grounding can create a noise transient offset voltage level between CGND and PGND. This could lead to faulty operation of the gate driver and MOSFETs. 10. Ringing at the BOOT pin is most effectively controlled by close placement of the boot capacitor. Do not add an additional BOOT to the PGND capacitor. This may lead to excess current flow through the BOOT diode. 11. The SMOD# and DISB# pins have weak internal pull-up and pull-down current sources, respectively. These pins should not have any noise filter capacitors. Do not to float these pins unless absolutely necessary. 12. Use multiple vias on the VIN and VOUT copper areas to interconnect top, inner, and bottom layers to distribute current flow and heat conduction. Do not put many vias on the VSWH copper to avoid extra parasitic inductance and noise on the switching waveform. As long as efficiency and thermal performance are acceptable, place only one VSWH copper on the top layer and use no vias on the VSWH copper to minimize switch node parasitic noise. Vias should be relatively large and of reasonably low inductance. Critical high- frequency components, such as RBOOT, CBOOT, RC snubber, and bypass capacitors; should be located as close to the respective DrMOS module pins as possible on the top layer of the PCB. If this is not feasible, they can be connected from the backside through a network of low-inductance vias. |
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