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LM20K Family PowerWise Synchronous Buck Regulator Slide 17

Here is an example of ratiometric sequencing. The value of R2 should generally be under 3.65Kohms to limit any errors due to the soft-start charging current in the track pin. Once R2 is selected, R1 can be calculated by the given formula. The formula is derived from the fact that the voltage at the track pin should equal the feedback voltage of Vout2 when Vout1 reaches its final value. Note that when calculating the value of R1, the highest expected voltage for Vfb2 and the lowest expected values for Vout1 and R2 should be used. If these tolerances are not taken into account, Vout2 may never reach its final value and any transient on Vout1 may show up on the output of Vout2. An extra 200mV is added to the value of Vfb2 to provide the needed overdrive on the SS/TRK pin. The extra 200mV will cause Vout2 to reach its final value slightly before Vout1. This should not be a problem for most applications that require ratiometric sequencing.

PTM Published on: 2011-11-02