TPS7A20: TPS7A20

Hi Tasneem,

If you aren't trying to regulate the output voltage, but instead just want to pass Vin to Vout with a delay, then it sounds like you need a load switch with soft start capability.  I'll move this to the power switches team for further guidance.

https://www.ti.com/power-management/power-switches/load-switches/products.html#546=Adjustable%20Rise%20Time&

Thanks,

Stephen

Thank you so much, Stephen! Yes, if someone can please help me find a chip that would suit my design problem, it would be a great help! 

Hi Tasneem,

Are you looking for something that will have a controlled VOUT slew rate during startup, or something more like a buffer with delay?

Thanks,

Patrick

Hi Patrick, I'm looking for something more of a buffer with a delay. Oh, I should note that there is no enable signal for me to use to turn the switch on. So, if I were to for example, use TPS22990 or TPS22997, I would want to tie both the input +5V signal to both VBIAS & ENABLE pins.  

Hi Tasneem,

The TPS22997 and TPS22990 (as well as the other load switches) will only create a variable output voltage ramp rate when the device starts up, so I don't think this exactly fits your application.

What delay are you looking for? And how accurate does this delay need to be? I have a few ideas for how to address this.

Also, we can discuss this in messages if you prefer.

Thanks,

Patrick

Thank you for the quick response. A delay of ~5 ms is sufficient.

This looks great, the only issue is that the max current is too low. Is there another chip that has a higher max output current? I'm looking for something on the order of 300 mA to 500 mA. Can I do the same thing with this chip: https://www.ti.com/lit/ds/symlink/tps7a20.pdf?ts=1692142837748 

Hi Tasneem,

Can you confirm that this drawing represents what you need the circuit to do?  If so, and if Vin takes a very long time to ramp up, then the RC time constant approach may be problematic as the RC time constant will not dominate the charging up of the capacitor.

Are you open to adding a sequencer to your design?  If you use something like the LM3880, you can set the delay time with a capacitor then the output of the sequencer (LM3880 or equivalent) will toggle low to high, enabling a load switch or LDO.  We can loop in the owners of the sequencer IC's to confirm the LM3880 is the right choice or if there is another option.

https://www.ti.com/lit/ds/symlink/lm3880.pdf

Thanks,

Stephen

Hi Stephen, Thank you for the diagram. I would say ideally, it would take 5 ms from Vin = 0 to Vin = 5V. Interestingly enough, I did look into using the LM3880. However, it's max output current is too low (50 mA). I then look into using a FET to amplify the current at each output but that turned out to be too complicated since I would need to fine tune each FET to feed each of my amplifiers(I have 2 on my board). 

Hi Tasneem,

This is what I'm referring to.  You can also replace the TPS7A20 with a load switch of your choosing (or with Patrick's support) in place of the TPS7A20 in this schematic.  The LM3880 will allow you to trigger the FLAG pins when Vin reaches a certain threshold (such as 95% of regulation).  Or you can program the delay using a capacitor on the EN pin of the LM3880. 

If you use resistor dividers from Vin to GND on the EN pin of the LM3880, you can set one of the FLAG pins to toggle at 95% of when Vin reaches regulation.  The TPS7A20 does not have a configurable soft start and it turns on it approximately 1ms as an exponential rise.  This is how the waveforms would look - notice that by using the LM3880 you do not need to worry about how long the Vin source takes to turn on since we are programming the FLAG pins.

Vout will not exactly equal Vin due to the device being in dropout.  I would recommend a load switch with programmable soft start to complement the LM3880.  Then you use the LM3880 to program the enable of the load switch and the programmable soft start of the load switch (usually just an extra capacitor) programs the turn on of the load switch.  This should meet your requirements.

Thanks,

Stephen

Thank you so much, Stephen and Patrick! I greatly appreciate this. Based on your idea of using a load switch, I chose to cascade the LM3880 with the TPS22990. So, it would look like this: 

Basically, I have only a single +5V line which I use in multiple pins but I think this should give me a 10 ms delay at 5V with a total output current of ~200 mA for me to feed each of my LNAs. 

Links to datasheets:

LM3880: https://www.ti.com/product/LM3880?utm_source=google&utm_medium=cpc&utm_campaign=app-null-null-GPN_EN-cpc-pf-google-wwe&utm_content=LM3880&ds_k=LM3880&DCM=yes&gad_source=1&gclid=EAIaIQobChMI0MHxiu7ZiQMV4VR_AB33wgewEAAYASAAEgJwb_D_BwE&gclsrc=aw.ds 

TPS22990: https://www.ti.com/product/TPS22990?utm_source=google&utm_medium=cpc&utm_campaign=app-null-null-gpn_en-cpc-pf-google-wwe&utm_content=tps22990&ds_k=TPS22990&dcm=yes&gad_source=1&gclid=EAIaIQobChMIq9fH-efZiQMVf0dHAR1rCy_zEAAYASAAEgKVn_D_BwE&gclsrc=aw.ds 

Great, thank you so much. Oh, I do want to correct myself in the schematic I sent, for the LM3880, the enable pin wouldn't be connected to Vcc since it is internally connect but if someone could please confirm that all the other pins are correct, I would greatly appreciate it since this is urgent? Thank you again.

Hi Stephen, 

  I was wondering if there was anyone to help me confirm the pin connections used in this way? Sorry to have to message again but this is very urgent. Thank you so much.