8.3.16.1 Operation

The TPS65217x step-down converters typically operate with 2.25-MHz fixed-frequency pulse-width modulation (PWM) at moderate-to-heavy load currents. At light load currents, the converter automatically goes to power-save mode and operates in pulse-frequency modulation (PFM).

During PWM operation, the converter uses a unique fast-response voltage-mode controller scheme with input-voltage feed-forward to achieve good line and load regulation. This controller scheme allows the use of small ceramic input and output capacitors. At the start of each clock cycle, the high-side MOSFET is turned on. The current flows from the input capacitor through the high-side MOSFET through the inductor to the output capacitor and load. During this phase, the current ramps up until the PWM comparator trips and the control logic turns off the switch. The current-limit comparator also turns off the switch in case the current limit of the high-side MOSFET switch is exceeded. After a dead time to prevent shoot-through current, the low-side MOSFET rectifier is turned on and the inductor current ramps down. The direction of current flow is now from the inductor to the output capacitor and to the load. The current returns back to the inductor through the low-side MOSFET rectifier.

The next cycle turns off the low-side MOSFET rectifier and turns on the on the high-side MOSFET.

The DC/DC converters operate in synchronization with each other, with converter 1 as the master. A 120° phase shift between DCDC1 and DCDC2 and between DCDC2 and DCDC3 decreases the combined input root mean square (RMS) current at the VIN_DCDCx pins. Therefore, smaller input capacitors can be used.