The TPS53211 is a Single Phase Voltage Mode Control PWM controller for Synchronous BUCK (Step-down) applications. At load currents below critical conduction (InVgductor peak-peak ripple current / 2) the TPS53211 switches to operate as an adaptive Constant On-time control topology to reduce the switching frequency and increase efficiency, but how should the MOSFETs be sized and are there limitations to the MOSFETs that should be used?
As with any converter design, MOSFETs should be selected to optimize size, cost and performance. Ideally, the MOSFETs are selected to balance switching losses and conduction losses at the load current of greatest interest to the application, or at most, the maximum load current.
Larger MOSFETs with lower Rdson and higher gate charge will have higher switching losses, decreasing efficiency.
Smaller MOSFETs with higher Rdson and lower gate charge will have higher conduction losses, also decreasing efficiency.
For most applications, selecting MOSFETs to follow this optimization should be sufficient.
However, TI recommends that the loading current on the VCCDR regulator (Total gate capacitance of both high-side and low-side FETs times the switching frequency) should be less than 50mA to avoid driving the VCCDR regulator into current limit.
In addition, TI recommends that if the Total gate charge (Qg) of the low-side FET should not exceed 50nC at 6.5V Vgs. If a low-side FET with greater gate charge must be used, TI recommends designers limit the LGATE discharge current with a 1.0Ω series resistor.
If multi MOSFETs are used in parallel, a separate gate drive resistor should be used for each MOSFET and the parallel combination of all gate resistors should be equal to or greater than 1.0Ω
