SNVSBJ0A December 2019 – June 2020 LM5170
The LM5170 requires an external 10- to 12-V VCC bias supply to operate. If not available in the system, the user can generate it from the LV-port using a buck-boost or SEPIC converter, or from the HV-port using a buck converter. Refer to the Texas Instruments LM25118-Q1 and LM5118-Q1 to implement a buck-boost converter, or LM5001-Q1 to implement a SEPIC converter, or the LM5160-Q1 and LM5161-Q1 to implement a buck converter.
The total load current of the bias supply is mainly determined by the total MOSFET gate charge Qg. Assume the system employs multiple LM5170s to implement M number of phases, and each phase uses N number of MOSFETs in parallel as one switch. There are 2× N MOSFETs per phase to drive. Then the total current to drive these MOSFETs through VCC bias supply is determined by Equation 60.
In an example of a four-phase system employing two parallelled MOSFETs for one switch, where M = 4, N = 2, Qg = 100 nC, and Fsw = 100 KHz, the bias supply should be able to support at least the following total load current:
In an example of an eight-phase system employing the same parallel MOSFETs for one switch, the bias supply should be able to support the following total load current:
The VCC AC bypass ceramic capacitor CVCC = 1 to approximately 2.2 µF, rated at least 16 V, must be placed close to the VCC and PGND pins. Similarly, a ceramic capacitor CVCCA = 1 µF, rated at least 16 V, must be placed close to the VCCA and AGND pins. Place a 24-Ω resistor between VCC and VCCA pins.