SNVAA91 December 2023 TPSM63610

3 Dynamic Output Voltage Control Method

Figure 3-1 presents a simple approach to dynamically control the output voltage of a DC/DC step down buck converter through multiple parallel lower feedback resistors and switches. Controlling the switch can place the external lower resistors in parallel with R_BOT to dynamically change the output voltage regulation point. Refer to Equation 1 for details on calculating the new output voltage. Though easy to implement, this control method has several drawbacks as listed below:

Abrupt resistor switching to adjust V_OUT from the lowest voltage to the highest voltage can cause the feedback node voltage to spike instantaneously
The component design for multiple output voltages increase due to the additional lower feedback resistors and switches required to dynamically adjust V_OUT. One way to minimize the switch components is to use a TS5A3359 (3:1, 1-channel analog multiplexer).
Additionally, if the system application requires slew rate control, refer to Section 9.1.6 of the voltage transition requirements section from the TPS25740B data sheet for more details on the additional slew rate control circuitry during negotiation of output voltage transitions. The design example in this app note does not cover this additional circuitry.

Figure 3-1 Feedback Resistor Switch Network Control Scheme

Equation 1.

V_{O U T} = V_{F B} \times (1 + \frac{R_{T O P}}{\frac{R_{B O T} \times R_{i}}{R_{B O T} + R_{i}}})

Note: R_i represents the external lower resistor (R₁, R₂, or R₃) that is placed in parallel with R_BOT through control of the individual MOSFET switches.

For a TI USB Type-C dual buck reference design example using this feedback resistor switch network control method, refer to PMP22416.