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L6747A Datasheet(PDF) 11 Page - STMicroelectronics |
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L6747A Datasheet(HTML) 11 Page - STMicroelectronics |
11 / 15 page L6747A Device description and operation Doc ID 17126 Rev 1 11/15 Referring to Figure 6, a classic MOSFET driver can be represented by a push-pull output stage with two different MOSFETs: a P-MOSFET to drive the external gate high, and an N- MOSFET to drive the external gate low (with their own RDS(on): Rhi_HS, Rlo_HS, Rhi_LS, Rlo_LS). The external power MOSFET can be represented in this case as a capacitance (CG_HS, CG_LS) that stores the gate-charge (QG_HS, QG_LS) required by the external power MOSFET to reach the driving voltage (PVCC for HS and VCC for LS). This capacitor is charged and discharged at the driver switching frequency FSW. The total power PSW is dissipated among the resistive components distributed along the driving path. According to the external gate resistance and the power MOSFET intrinsic gate resistance, the driver dissipates only a portion of PSW as follows: The total power dissipated from the driver can then be determined as follows: Figure 6. Equivalent circuit for a MOSFET driver 4.5 Layout guidelines The L6747A provides driving capability to implement high-current step-down DC-DC converters. The first priority when placing components for these applications should be given to the power section, minimizing the length of each connection and loop as much as possible. To minimize noise and voltage spikes (as well as EMI and losses) power connections must be part of a power plane, and in any case constructed with wide and thick copper traces. The loop must be minimized. The critical components, such as the power MOSFETs, must be close to each other. However, some space between the power MOSFETs is required to assure good thermal cooling and airflow. P SW HS – 1 2 --- C GHS PVCC 2 Fsw R hiHS R hiHS R GateHS R iHS ++ ---------------------------------------------------------------- R loHS R loHS R GateHS R iHS ++ ---------------------------------------------------------------- + ⎝⎠ ⎛⎞ ⋅⋅ ⋅ ⋅ = P SW LS – 1 2 --- C GLS VCC 2 Fsw R hiLS R hiLS R GateLS R iLS ++ -------------------------------------------------------------- R loLS R loLS R GateLS R iLS ++ -------------------------------------------------------------- + ⎝⎠ ⎛⎞ ⋅⋅ ⋅ ⋅ = PP DC P SW HS – P SW LS – ++ = RGATELS RILS CGLS VCC LS DRIVER LS MOSFET GND LGATE RGATEHS RIHS CGHS BOOT HS DRIVER HS MOSFET PHASE HGATE VCC |
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