LT8642S Parallel 20A with soft start and mode control


We intend to connect two LT8642S in parallel as explained in the applications information section of the datasheet.

However, there is some confusion as to how to connect some of the pins.

1. TR/SS Soft Start

Should the TR/SS pin of each regulator be connected together with a single capacitor controlling the rise time?
If not, how should the two pins be connected in order to enable soft start in a parallel configuration?

2. Sync/Mode

We would also like to be able to control the switching frequency or toggle spread spectrum mode on and off.
When there is only 1x LT8642S, we have control over its modes because we can connect SYNC/MODE to the 3.3V bank of our FPGA.

  • Drive low = burst mode
  • Drive at frequency = synchronisation mode.
  • Drive high = spread spectrum mode.

We would like to keep these three options with the parallel configuration. However, the secondary LT8642S is clocked by the CLKOUT pin of the primary regulator and is not controlled by the FPGA.
When the FPGA puts the primary LT8642S into burst mode or spread spectrum mode, what will the secondary LT8642S do?

3. PG Power Good

Is it possible to connect the two PG pins together? We would prefer not to implement an AND gate in order to enable the subsequent circuits.

Kind regards,


  • +1
    •  Analog Employees 
    on Aug 28, 2020 6:03 AM

    Hi Dan,

    1. TR/SS soft start

    Connect TR/SS pins of each regulator together for the same startup time in parallel application.

    1. Sync/mode

    To sync two LT8642S in parallel applications, CLKOUT of one LT8642S (U1) usually is connected to SYNC/MODE of the other LT8642S (U2). In this way, U2 will follow U1 frequency, no matter it is fixed or has variation (such as spread spectrum). Note that two LT8642S can’t be synced in burst mode, since CLKOUT doesn’t have clock output in burst mode.

    1. PG Power Good

    Two PG pins could be tied together in parallel application.