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LM27222SDX Scheda tecnica(PDF) 8 Page - National Semiconductor (TI) |
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LM27222SDX Scheda tecnica(HTML) 8 Page - National Semiconductor (TI) |
8 / 11 page Application Information (Continued) POWER DISSIPATION The power dissipated in the driver IC when switching syn- chronously can be calculated as follows: where f SW = switching frequency V CC = voltage at the VCC pin, Q G_H = total gate charge of the (parallel combination of the) high-side MOSFET(s) Q G_L = total gate charge of the (parallel combination of the) low-side MOSFET(s) R G_H = gate resistance of the (parallel combination of the) high-side MOSFET(s) R G_L = gate resistance of the (parallel combination of the) low-side MOSFET(S) R H_pu = pull-up RDS_ON of the high-side driver R H_pd = pull-down RDS_ON of the high-side driver R L_pu = pull-up RDS_ON of the low-side driver R L_pd = pull-down RDS_ON of the low-side driver PC BOARD LAYOUT GUIDELINES 1. Place the driver as close to the MOSFETs as possible. 2. HG, SW, LG, GND: Run short, thick traces between the driver and the MOSFETs. To minimize parasitics, the traces for HG and SW should run parallel and close to each other. The same is true for LG and GND. 3. Driver V CC: Place the decoupling capacitor close to the V CC and GND pins. 4. The high-current loop between the high-side and low- side MOSFETs and the input capacitors should be as small as possible. 5. There should be enough copper area near the MOS- FETs and the inductor for heat dissipation. Vias may also be added to carry the heat to other layers. TYPICAL APPLICATION CIRCUIT DESCRIPITON The Application Example on the following page shows the LM27222 being used with National’s LM27212, a 2-phase hysteretic current mode controller. Although this circuit is capable of operating from 5V to 28V, the components are optimized for an input voltage range of 9V to 28V. The high-side FET is selected for low gate charge to reduce switching losses. For low duty cycles, the average current through the high-side FET is relatively small and thus we trade off higher conduction losses for lower switching losses. The low-side FET is selected solely on R DS_ON to minimize conduction losses. If the input voltage range were 4V to 6V, the MOSFET selection should be changed. First, much lower voltage FETs can be used, and secondly, high-side FET R DS_ON becomes a larger loss factor than the switching losses. Of course with a lower input voltage, the input ca- pacitor voltage rating can be reduced and the inductor value can be reduced as well. For a 4V to 6V application, the inductor can be reduced to 200nH to 300nH. The switching frequency of the LM27212 is determined by the allowed ripple current in the inductor. This circuit is set for approxi- mately 300kHz. At lower input voltages, higher frequencies are possible without suffering a significant efficiency loss. Although the LM27222 can support operating frequencies up to 2MHz in many applications, the LM27212 should be lim- ited to about 1MHz. The control architecture of the LM27212 and the low propagation times of the LM27222 potentially gives this solution the fastest transient response in the industry. www.national.com 8 |
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