In reference to this datasheet, if you see pg2 has the typical application and pg12 has the layout guide. Why would the BST/SS/TON pins be shown unconnected compared to the typical application? They show the other pins/components wired up, but not BST/SS/TON.

Also, can I leave those three pins unconnected?

Here are the images from the mentioned screenshot for reference: Typical Application Layout Guide

  • 1
    \$\begingroup\$ the paragraph on page 12 explains the purpose of the layout diagram ... it is not meant to cover control pins \$\endgroup\$
    – jsotola
    Dec 6, 2019 at 4:02
  • \$\begingroup\$ I’m having a hard time figuring out the TON pin. I have about 12v on the input and will have 5v on the output, and I’m struggling to figure out what roll the TON pin plays. I read the sections in the data sheet that explained it and googled terms and concepts I didn’t know, but I can’t wrap my head around it’s use. I want the light load feature this chip offers, but it seems if I set and an on time with Rton I will lose the light load capability? \$\endgroup\$
    – user164080
    Dec 6, 2019 at 4:16

2 Answers 2


There is a reason why datasheet has multiple sections and that is to give as much as details about their part.

First page covers about its key features. Also they have added application circuit which is nothing but kind of schematic using their PWM BUCK converter.

Now, when you are using SMPS in your design, you should give great attention to the layout techniques. that's why usually manufacturer recommends layout technique for their IC. main popular layout techniques are,

  • Provide closure path from input capacitor GND to output capacitor GND for non isolated power converter design.
  • Keep high di/dt and dv/dt regions smaller to the IC
  • Avoid routing sensitive signals under switching planes etc..

to help buyers, manufacturer added important layout technique and it should be followed most of the time.

Now, lets come to SS/BST/Ton. Nothing is provided without any reason. there is a specific reason. you cannot keep open without connecting anything into it.

SS is stands for soft start and most of the SMPS has this feature. User can connect an capacitor to limits its start-up inrush current and also to avoid overshoot at output during start-up.

BST is stands for Bootstrap and it is needed for driving high side mosfet. it should be connected with a capacitor and some time diode as well to make the buck converter operational.

TON is the special feature of this IC where they claims to keep constant on time always when the input voltage changes and eventually this helps to achieve faster transient response time of the converter.

My advice would please read a lot of power converter techniques and then starts to implement. Just doing google wont help much without proper understanding or eager to learn something. good luck.


Firstly, schematic is just one small portion of the entire design process, the designers have a higher obligation of creating PCB designs that can actually make the ideas sell.

Having said that, layout guidelines provided in datasheets, typically emphasize PCB layout considerations that would affect the performance of the application. Thermal and electrical characteristics of a device highly affect the performance of the system in which they are used. The layout guidelines provides information to achieve optimum thermal and electrical performance from the device.

The example mentioned in this case, is a dc-dc converter. The layout tips provided, are majorly for the energy storage elements like inductors and capacitors, which generally dissipates heats and has fast switching actions. A poor layout for these components will majorly affect the thermal and electrical characteristics of the device, thereby affecting the performance of the system in which it is being used.


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