SNVSB29C October 2018 – June 2021 LM5143-Q1
There are two limitations to the minimum output voltage adjustment range: the LM5143-Q1 voltage reference of 0.6 V and the minimum controllable switch-node pulse width, tON(min).
tON(min) effectively limits the voltage step-down conversion ratio of VOUT/VIN at a given switching frequency. For fixed-frequency PWM operation, the voltage conversion ratio must satisfy Equation 8.
If the desired voltage conversion ratio does not meet the above condition, the LM5143-Q1 transitions from fixed switching frequency operation to a pulse-skipping mode to maintain output voltage regulation. For example, if the desired output voltage is 5 V with an input voltage is 24 V and switching frequency of 2.1 MHz, the voltage conversion ratio test in Equation 9 is satisfied.
For wide VIN applications and low output voltages, an alternative is to reduce the LM5143-Q1 switching frequency to meet the requirement of Equation 8.