TPS3420: How to guarentee that Ttimer is never above 8s

Cameron Wutzke

Part Number: TPS3420
Other Parts Discussed in Thread: TPS3899

Hi team!

Using the TPS3420 as a reset pushbutton where a reset is performed by holding the power button for about 10s typically. Sometimes, there is very little current drawn on Vsys, so the rail stays high and chips are not power reset over this 10s.

This is where the TPS3420 looks ideal. it can discharge the Vsys rail after the power button has been held for many seconds.

The TPS3420 looks like an ideal solution but the maximum specification is 9s. In this application, Ttimer can never be above 8s. IF above 8s, we miss a critical window where battery FET is disconnected.

Any thoughts or possible solutions around this? We are open to chip recommendations or circuit ideas

Thanks,

Cameron

over 1 year ago

0 Joshua Austria over 1 year ago

TI__Expert 6395 points

Hi Cameron,

Apologies as I am a little confused as to exactly what specs you want for a timer. If you want a device with a Ttimer above 10s or with a Ttimer below 8s, you can use the TPS3420DDRYR/T. Here is the associated table for reference:

Hope this helps,

Joshua Austria

0 Cameron Wutzke over 1 year ago in reply to Joshua Austria

TI__Intellectual 2271 points

Hey Joshua,

That 7.5ms is the typical value which is perfect. If the device is performs to the typ spec, it will work perfectly.

We are worried about the edge case where the max for tTimer is 9s:

0 Cameron Wutzke over 1 year ago in reply to Cameron Wutzke

TI__Intellectual 2271 points

I'll add a little more flavor too. This is a super power sensitive application. Where every uA matters. So the goal is to avoid wasting any power for the first 6ish seconds while the power button is held. After those 6ish seconds, we want to drain the power rail. So the idea is to use a load circuit to drain the Vsys rail to reset every chip connected.

There's a battery charger in this application which will disconnect the battery from vsys after a button is pressed for 8-12 seconds (min to max spec). The disconnect happens for 300ms so we need to make sure the load circuit is active while this disconnect happens.

This is where the concern comes in: what if we push the button. 8s later, the battery disconnects, but the TPS3420 does not trigger until 9s. This assumes worst case scenario for both chips