8.3.2 Step-Down Converters, VDCDC1, VDCDC2, and VDCDC3

The TPS65023x incorporates three synchronous step-down converters operating typically at
2.25-MHz, fixed frequency pulse width modulation (PWM) at moderate to heavy-load currents. At light-load currents, the converters automatically enter the power save mode (PSM), and operate with pulse frequency modulation (PFM). The VDCDC1 converter is capable of delivering 1.5-A output current, the VDCDC2 converter is capable of delivering 1.2 A and the VDCDC3 converter is capable of delivering up to 1 A.

The converter output voltages can be programmed through the DEFDCDC1, DEFDCDC2, and DEFDCDC3 pins. The pins can either be connected to GND, VCC, or to a resistor divider between the output voltage and GND. The VDCDC1 converter defaults to 1.2 V or 1.6 V depending on the DEFDCDC1 configuration pin. If DEFDCDC1 is tied to ground, the default is 1.2 V. If it is tied to VCC, the default is 1.6 V. When the DEFDCDC1 pin is connected to a resistor divider, the output voltage can be set in the range of 0.6 V to VINDCDC1 V. See Application Information for more details. The core voltage can be reprogrammed through the serial interface in the range of 0.8 V to 1.6 V with a programmable slew rate. The converter is forced into PWM operation whilst any programmed voltage change is underway, whether the voltage is being increased or decreased. The DEFCORE and DEFSLEW registers are used to program the output voltage and slew rate during voltage transitions.

The VDCDC2 converter defaults to 1.8 V or 3.3 V depending on the DEFDCDC2 configuration pin. If DEFDCDC2 is tied to ground, the default is 1.8 V. If it is tied to VCC, the default is 3.3 V. When the DEFDCDC2 pin is connected to a resistor divider, the output voltage can be set in the range of 0.6 V to VINDCDC2 V.

The VDCDC3 converter defaults to 1.8 V or 3.3 V depending on the DEFDCDC3 configuration pin. If DEFDCDC3 is tied to ground the default is 1.8 V. If it is tied to VCC, the default is 3.3 V. When the DEFDCDC3 pin is connected to a resistor divider, the output voltage can be set in the range of 0.6 V to VINDCDC3 V.

The step-down converter outputs (when enabled) are monitored by power-good (PG) comparators, the outputs of which are available through the serial interface. The outputs of the DC-DC converters can be optionally discharged through on-chip 300-Ω resistors when the DC-DC converters are disabled.

During PWM operation, the converters use a unique fast response voltage mode controller scheme with input voltage feedforward to achieve good line and load regulation allowing the use of small ceramic input and output capacitors. At the beginning of each clock cycle initiated by the clock signal, the P-channel MOSFET switch is turned on. The inductor current ramps up until the comparator trips and the control logic turns off the switch. The current limit comparator also turns off the switch if the current limit of the P-channel switch is exceeded. After the adaptive dead-time used to prevent shoot through current, the N-channel MOSFET rectifier is turned on, and the inductor current ramps down. The next cycle is initiated by the clock signal, again turning off the N-channel rectifier and turning on the P-channel switch.

The three DC-DC converters operate synchronized to each other with the VDCDC1 converter as the master. A 180° phase shift between the VDCDC1 switch turn on and the VDCDC2 and a further 90° shift to the VDCDC3 switch turn on decreases the input RMS current and smaller input capacitors can be used. This is optimized for a typical application where the VDCDC1 converter regulates a Li-Ion battery voltage of 3.7 V to 1.2 V, the VDCDC2 converter from 3.7 V to 1.8 V, and the VDCDC3 converter from 3.7 V to 3.3 V. The phase of the three converters can be changed using the CON_CTRL register.