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L6747A Datasheet(PDF) 10 Page - STMicroelectronics |
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L6747A Datasheet(HTML) 10 Page - STMicroelectronics |
10 / 15 page Device description and operation L6747A 10/15 Doc ID 17126 Rev 1 The bootstrap capacitor value should be selected to obtain a negligible discharge due to the turning on of the high-side MOSFET. It must provide a stable voltage supply to the high-side driver during the MOSFET turn-on, and minimize the power dissipated by the embedded boot diode. Figure 5 illustrates some guidelines on how to select the capacitance value for the bootstrap according to the desired discharge, and the selected MOSFET. To prevent the bootstrap capacitor from overcharging as a consequence of large negative spikes, an external series RBOOT resistor (in the range of few ohms) may be required in series with the BOOT pin. Figure 5. Bootstrap capacitance design 4.4 Power dissipation The L6747A embeds high current drivers for both high-side and low-side MOSFETs. It is therefore important to consider the power that the device is going to dissipate in driving them in order to avoid exceeding the maximum junction operating temperature. Two main factors contribute to device power dissipation: bias power and driver power. ● Device power (PDC) depends on the static consumption of the device through the supply pins and is easily quantifiable as follows: ● Driver power is the power needed by the driver to continuously switch the external MOSFETs ON and OFF. It is a function of the switching frequency and total gate charge of the selected MOSFETs. It can be quantified considering that the total power PSW dissipated to switch the MOSFETs is influenced by three main factors: external gate resistance (when present), intrinsic MOSFET resistance, and intrinsic driver resistance. This last factor is the important one to be determined to calculate the device power dissipation. The total power dissipated to switch the MOSFETs is: When designing an application based on the L6747A, it is recommended to take into consideration the effect of external gate resistors on the power dissipated by the driver. External gate resistors help the device to dissipate the switching power since the same power PSW is shared between the internal driver impedance and the external resistor, resulting in a general cooling of the device. P DC V CC I CC V PVCC I PVCC ⋅ + ⋅ = P SW F SW Q GHS PVCC ⋅ Q GLS VCC ⋅ + () ⋅ = |
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