JAJSB34I 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 RON value.
This can be rearranged as
The selection of RON and fSW(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 RON and the input voltage VIN. It is calculated as follows:
The inverse relationship of tON and VIN gives a nearly constant switching frequency as VIN is varied. RON should be selected such that the ON-time at maximum VIN is greater than 150 ns. The ON-timer has a limiter to ensure a minimum of 150 ns for tON. This limits the maximum operating frequency, which is governed by Equation 15:
This equation can be used to select RON if a certain operating frequency is desired so long as the minimum ON-time of 150 ns is observed. The limit for RON can be calculated as follows:
If RON calculated in Equation 13 is less than the minimum value determined in Equation 16 a lower frequency should be selected. Alternatively, VIN(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 RON (lower FSW) should be selected in any application requiring large duty ratio.