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L6728A Datasheet(PDF) 10 Page - STMicroelectronics |
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L6728A Datasheet(HTML) 10 Page - STMicroelectronics |
10 / 32 page L6728A Driver section 10/32 5 Driver section The integrated high-current drivers allow using different types of power MOSFET (also multiple MOSFETs to reduce the equivalent RDS(on)), maintaining fast switching transition. The driver for the high-side MOSFET uses BOOT pin for supply and PHASE pin for return. The driver for low-side MOSFET uses the VCC pin for supply and GND pin for return. The controller embodies an anti-shoot-through and adaptive dead-time control to minimize low side body diode conduction time, maintaining good efficiency while saving the use of Schottky diode: to check high-side MOSFET turn off, PHASE pin is sensed. When the voltage at PHASE pin drops down, the low-side MOSFET gate drive is suddenly applied; to check low-side MOSFET turn off, LGATE pin is sensed. When the voltage at LGATE has fallen, the high-side MOSFET gate drive is suddenly applied. If the current flowing in the inductor is negative, voltage on PHASE pin will never drop. To allow the low-side MOSFET to turn-on even in this case, a watchdog controller is enabled: if the source of the high-side MOSFET doesn't drop, the low side MOSFET is switched on so allowing the negative current of the inductor to recirculate. This mechanism allows the system to regulate even if the current is negative. Power conversion input is flexible: 5 V, 12 V bus or any bus that allows the conversion (See maximum duty cycle limitations) can be chosen freely. 5.1 Power dissipation L6728A embeds high current MOSFET drivers for both high side and low side MOSFETs: it is then important to consider the power that the device is going to dissipate in driving them in order to avoid overcoming the maximum junction operative temperature. Two main terms contribute in the device power dissipation: bias power and drivers' power. ● Device bias power (PDC) depends on the static consumption of the device through the supply pins and it is simply quantifiable as follow (assuming to supply HS and LS drivers with the same VCC of the device): ● Drivers power is the power needed by the driver to continuously switch on and off the external MOSFETs; 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 (easy calculable) is dissipated by three main factors: external gate resistance (when present), intrinsic MOSFET resistance and intrinsic driver resistance. This last term is the important one to be determined to calculate the device power dissipation. The total power dissipated to switch the MOSFETs results: External gate resistors helps the device to dissipate the switching power since the same power PSW will be shared between the internal driver impedance and the external resistor resulting in a general cooling of the device. P DC V CC I CC I BOOT + () ⋅ = P SW F SW Q gHS V BOOT Q gLS V CC ⋅ + ⋅ () ⋅ = |
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