SNVS690I January 2011 – August 2021 LMZ14201H
PRODUCTION DATA
Many designs will begin with a desired switching frequency in mind. As seen in the Typical Characteristics section, the best efficiency is achieved in the 300 kHz to 400 kHz switching frequency range. Equation 12 can be used to calculate the R_{ON} value.
This can be rearranged as
The selection of R_{ON} and f_{SW(CCM)} must be confined by limitations in the ON-time and OFF-time for the COT Control Circuit Overview section.
The ON-time of the LMZ14201H timer is determined by the resistor R_{ON} and the input voltage V_{IN}. It is calculated as follows:
The inverse relationship of t_{ON} and V_{IN} gives a nearly constant switching frequency as V_{IN} is varied. R_{ON} should be selected such that the ON-time at maximum V_{IN} is greater than 150 ns. The ON-timer has a limiter to ensure a minimum of 150 ns for t_{ON}. This limits the maximum operating frequency, which is governed by Equation 15:
This equation can be used to select R_{ON} if a certain operating frequency is desired so long as the minimum ON-time of 150 ns is observed. The limit for R_{ON} can be calculated as follows:
If R_{ON} calculated in Equation 13 is less than the minimum value determined in Equation 16 a lower frequency should be selected. Alternatively, V_{IN(MAX)} can also be limited in order to keep the frequency unchanged.
Additionally, the minimum OFF-time of 260 ns (typical) limits the maximum duty ratio. Larger R_{ON} (lower F_{SW}) should be selected in any application requiring large duty ratio.