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Buck converter for the ISL8117 track an external voltage level

Slalom

Hello community, I'm working on a project about Buck Converter using ISL8117.

I'm using it with
Input voltage  Vin = 24V.
Output voltage Vout is adjustable. 
Vout_min = 3V.   Vout_max = 21V.      

The ISL8117 will be set up to track an external voltage level Vmaster.
Voltage level Vmaster is the output of external DAC.  
Vmaster_min = 0V.  Vmaster_max = 10V.
A resistive divider is connected between the Vmaster and ground.
The center point of the divider will be connected to the SS/TRK pin of the ISL8117.
The voltage at SS/TRK pin (Vtrk) is greater than 0.6V when the master rail reaches regulation.
Output voltage Vout = Vout_min =    3V, when Vtrk = 0,6V. 
Output voltage Vout = Vout_max = 21V, when Vtrk = 4,2V. 


What is the maximum voltage Vtrk at SS/TRK pin for tracking control mode ISL8117?
Does value Vtrk = 4,2V is legal level for tracking control mode ISL8117?

Thank you.

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  • 0 in reply to Slalom

    Hi, 

    Please below table from datasheet 

    Also ,set the tracking resistive divider ratio exactly the same as the ISL8117 output resistive divider. For your case its different. 

    Thanks 

    AB 

  • 0 in reply to AB_RENESAS

    Hello,

    Unfortunately, absolute maximum rating only tells me that the IC will not be damaged if we stay within the specified range.  I doubt very much that the tracking will happen as expected within whole this range of voltages on SS/TRK pin.  Therefore is my original question, which I have to repeat again.

    Moreover, looking on the figures 13 and 26 in the datasheet and based on the description of how Slow Start works, I tend to think now that the  description in the datasheet of how TRK works has a dramatic error.  It is written there that care shall be taken that TRK stays greater than 0.6V.  Whereas, it looks that it shall be NOT_greater than 0.6V.  Correspondingly, for tracking operation in means that SS/TRK pin has to be within 0-0.6V.  Can you please confirm this or disprove this statement?

    Thanks and best regards

  • 0 in reply to Slalom

    Hi,

    Looks like you purpose is to use controller’s SPI signal to control ISL8117’s output to track a voltage rail. For same  the Vmaster signal should adjust the FB pin voltage not SS/TRK to adjust ISL8117’s Vout..SS/TRK pin is not for this purpose . Kindly find below diagram as reference .R value deepends on the controller DA signal. maybe start with 10k,

     

    Thanks 

    AB