TPS62742: TPS62742 maximum output current

Dear Chris, 

Thanks a lot for your recommendation. 

I am checking the recommended part and will let you know if I have any queries. Does the recommended part offer true output disconnect feature? This is a must-have for my application.

So when the EN is pulled low, what will be the output voltage? 

With the current scenario, what's the maximum amount of current I can withdraw from TPS62742? 

In one case, I am withdrawing 70 mA at 2.8V driven by a single D cell H class battery at 3.65V. 

In the second case, I am withdrawing 120 mA at 3.3V driven by a single D cell H class battery at 3.65V. 

Are these safe/allowed operational ranges? 

Thanks 

Hi Awais,

The recommended maximum output current should be clear on page 5 of the D/S.  You should be able to achieve your written currents (even though one is above 100 mA), since you have a higher Vin level.

Yes, most all buck converters disconnect the output when off.  All these devices also feature output discharge to pull Vout to GND.

What type of battery(ies) is it and what is your voltage range?

Dear Chris, 

Thanks for your reply. 

For the 120 mA case, my input voltage is 3.65V by the battery and the supply is configured at 3.3V. However, this pulse is only for 1.5s max, and that too at module frequency transmission. The average is around 50mA. 

Is that acceptable? 

Please find attached the battery datasheetser34615h.pdf. 

Thanks 

Hi Awais,

You will need to check if this gives sufficient performance for your specific application.

One possible issue I see is that the battery's voltage drops due to its internal impedance.  You can see that the battery only gives about 3.3V when a 100 mA load is placed on it.  Then, there will be further voltage drops across the DC/DC and its inductor, so the voltage at the output will be below 3.3V.

Hi Chris, 

Thanks for your reply. 

Indeed and I have been testing the system's performance for a few months now. 

However, I want to test it further and analyze if the same system is stable enough to perform for a longer period of time; say seven years. 

Attached is a better alternative to the battery. 

I think a way forward would be to add a low Iq LDO dedicated for the 100 mA load (in the present case a microchip radio module) driven directly by the battery or use the supply you have recommended. 

Thanks for your insights. 

ER34615M Spiral D.pdf

Yes, that battery looks like it has lower internal impedance.

TI just released the lowest Iq DC/DC and LDO recently, in TPS62840 and TPS7A02.  If your application runs in standby mode a lot, they can help prolong the battery life.

Hi Chris, 

Thanks for your reply. 

Indeed my application runs on stand-by mode a lot and only draws 120 mA in peaks when transmitting. That too happens only twice or thrice at max during a 24 hr period. 

This might be one of the reasons the system has been stable even with an inert flaw. 

So maybe my system can remain stable with an M class battery but using tps62742 for an output voltage of 3.3V.

I will read more about the recommended parts and will let you know accordingly. 

Thanks

Hi Chris, 

Another thing that I wanted to clarify is the current limiting functionality when the Vin is less than Vout. 
In that case, Vout will be what the Vin is. Right? 

Will it also limit the current? 

e.g. My Vout is set 3.3V
Vin drops to 2.8V 

Then Vout should be 2.8V. Now, will it still limit the current to say 100mA? 

Thanks 

Hi Awais,

Yes, when Vin drops towards Vout, the IC will enter 100% mode and Vout will initially go up towards Vin.  This is explained in section 9.4.2 of the D/S--see figure 6.  In 100% mode, Vout will be lower than Vin by the IR drop across the inductor and high side FET.

Note that the current limit does not change in 100% mode--it is always active at its nominal level.  The 100 mA is a recommendation of what you can draw from the IC--but the IC will not limit the current to 100 mA.  After you've entered 100% mode, it is ok to draw up to 400 mA of load current.