For the example design, the R6 selection is 100 kΩ. Using Equation 33, calculate R7 as 80 kΩ. The nearest standard 1% resistor is 80.5 kΩ.

Equation 33.

Because of the internal design of the TPS54388C-Q1 device, there is a minimum output-voltage limit for any given input voltage. The output voltage can never be lower than the internal voltage reference of 0.8 V. Above 0.8 V, an output voltage limit may exist due to the minimum controllable on-time. In this case, Equation 34 gives the minimum output voltage:

Equation 34.

where

• V_O(min) = minimum achievable output voltage
• t_(ONmin) = minimum controllable on-time (65 ns typical, 120 ns with no load)
• f_(SWmax) = maximum switching frequency, including tolerance
• V_I(max) = maximum input voltage
• I_O(min) = minimum load current
• r_DS(on) = minimum high-side MOSFET on-resistance (15 mΩ–19 mΩ)
• R_(L) = series resistance of output inductor

There is also a maximum achievable output voltage, which is limited by the minimum off-time. Equation 35 gives the maximum output voltage.

Equation 35.

where

• V_O(max) = maximum achievable output voltage
• t_(OFFmax) = maximum off-time (60 ns, typical)
• f_(SWmax) = maximum switching frequency, including tolerance
• V_I(min) = minimum input voltage
• I_O(max) = maximum load current
• r_DS(on) = maximum high-side MOSFET on-resistance (19 mΩ–30 mΩ)
• R_(L) = series resistance of output inductor