JAJSJT8C May 2020 – June 2021 TPS543620
PRODUCTION DATA
The first step is to decide on a switching frequency. The TPS543620 can operate at five different frequencies from 500 kHz to 2.2 MHz. The f_{SW} is set by the resistor value from the FSEL pin to ground. Typically the highest switching frequency possible is desired because it produces the smallest solution size. A high switching frequency allows for smaller inductors and output capacitors compared to a power supply that switches at a lower frequency. The main tradeoff made with selecting a higher switching frequency is extra switching power loss, which hurts the efficiency of the regulator.
The maximum switching frequency for a given application can be limited by the minimum on-time of the regulator. The maximum f_{SW} can be estimated with Equation 4. Using the maximum minimum on-time of 40 ns and 13.2-V maximum input voltage for this application, the maximum switching frequency is 1890 kHz. The selected switching frequency must also consider the tolerance of the switching frequency. A switching frequency of 1000 kHz was selected for a good balance of solution size and efficiency. To set the frequency to 1000 kHz, the selected FSEL resistor is 11.8 kΩ per Table 7-1.
Figure 8-2 shows the maximum recommended input voltage versus output voltage for each FSEL frequency. This graph uses a minimum on-time of 45 ns and includes the 10% tolerance of the switching frequency. A minimum on-time of 45 ns is used in this graph to provide margin to the minimum controllable on-time to ensure pulses are not skipped at no load. At light loads, the dead-time between the low-side MOSFET turning off and high-side MOSFET turning on contributes to the minimum SW node pulse-width.
In high output voltage applications, the minimum off-time must also be considered when selecting the switching frequency. When hitting the minimum off-time limits, the operating duty cycle will max out and the output voltage will begin to drop with the input voltage. Equation 5 calculates the maximum switching frequency to avoid this limit. This equation requires the DC resistance of the inductor, R_{DCR}, selected in the following step. A preliminary estimate of 10 mΩ can be used but this should be recalculated based on the specifications of the inductor selected. If operating near the maximum f_{SW} limited by the minimum off-time, the increase in resistance at higher temperature must be considered.