LDO with very low Iq and Vdrop suggestion

Hi Ilya,

Is the 2A current draw pulsed, or steady state?  If it is steady state and if you use an LDO for this, you will dissipate (5.1V - 3.4V) * 2A = 3.4W.  This is a lot of power for an LDO so you will want excellent thermal performance of the PCB.  If this power dissipation is excessive then it would be better for you to use a switching regulator. 

On the other hand, if this power loss is acceptable or if it is a pulsed load (so average power loss is low) then let me know and we can offer some solutions.

Thanks,

Stephen

Hello Stephen

My load is a "pulsed load" - 2A is for Secular Modem 2G, my average consumption is 300mA.

I don't know what power solution is good for me - switching regulator or LDO.

What's very important to me, my source is a battery and I need very low drop voltage and very low Iq.

Thank you

Ilya

Hello Stephen

Thank you very much for your suggestion, it look very good !!!

1. I would like ask question from professional - for my application what is better to implement LDO or DC/DC?

My device 80% under sleep and consumption is 100uA and that's my significant consumption from the battery.

2. what a difference between TPS7A52 and TPS7A92?

3. unfortunately, i am not found what a quiescent current of TPS7A52 when Vin=5.5, Ven="1" and current output is 100uA@3.3V?

4. how can i get samples of TPS7A52 or TPS7A92 ? no you, TI web side suggest and no DigiKey. How can i get 20 samples for prove of concept, POC.

Best regards

Ilya

Hi Ilya,

I am reviewing your question and will reply within 2 business days.

Thanks,

Stephen

Hi Ilya,

1. Unfortunately we cannot recommend either a DC/DC converter or linear regulator here, as the customer knows their application better than we do.  Having said that, customers will often move towards an LDO regulator if they have battery operation with low average power.  You can try a linear regulator here to start with, and if you find that it does not work for you then you can try a DC/DC converter.

2. Some key differences between the two:

3. I will need to take that measurement, which I can do but I will need a few business days to obtain an EVM.

4. I will add a colleague to this discussion who can better assist with sample requests.

Thanks,

Stephen

Hello Stephen

Thank you again for clear explanation.

Regarding to DC/DC converter or LDO for may application.

1. I would like to learn from your :) -- why for LDO regulator for battery operation and low average power is better solution then DC/DC low average power.

2. Do I understand that the quiescent current at low average power - LDO suggests better performance?

3. Please help me to understand compare table PS7A52 and TPS7A92, or what important parameters i need to tack or pay attention from LDO or DC/DC for battery operation with low average power?

4. From your table of differences between PS7A52 and TPS7A92 i see that Vdo on TPS7A52 is much better. I think that parameter very important for battery power application, I am right ?

5. Why in the datasheet of TPS7A52 do not suggest graphic of quiescent current vs output current? can you help me with that? quiescent current output is 100uA@3.3V?

Best regards

Ilya

Hi Wilson

Nice to meet you

what is mean --> I don't believe we have anymore of the TPS7A52

Is TPS7A52 End-of-life product?

Which one a new component TPS7A52 and TPS7A92?

what is DM - If you can DM me? 

How many of TPS7A53 do you have, i need 15 samples. 

Best regards

Ilya

Hi Ilya,

As the customer knows their application better, it will be up to you to determine if an LDO is a better fit than a DC/DC converter.  Some items you may wish to continue:

A. Power dissipation - at heavier loads, the DC/DC is likely more efficient than the LDO, but this is only 30% of the time.  At lighter loads the LDO may be more efficient as it possibly requires less current draw.

B. Complexity - LDO's typically require much less effort and design time to complete than DC/DC converters

C. Space - LDO's typically take up much less space than DC/DC converters

D. EMI compliance - an input filter may be needed to filter out the DC/DC switching frequency to maintain EMI compliance.  An LDO typically does not have this constraint.

For these reasons and the low average power, it may be a good idea to start with the less complex solution first, then if that does not work you can justify the more complex solution next.

With regards to your other questions:

2. It depends on the linear regulator and DC/DC converter performance comparison.  You would need to run a trade study on LDO's and DC/DC converters to determine which has better "performance".  Performance can mean multiple things to multiple customers, so this is something the customer will have to review for their specific application.

3. The top concerns for a low dropout regulator (LDO) would be quiescent current, dropout voltage, and maximum output current.  The LDO should meet the system needs for accuracy on Vout.  Secondary needs are PSRR and noise, unless this is an absolutely critical noise application.  As you are pulsing the load current, you must have very good transient response to stay within a tolerance requirement on Vout.  You may wish for additional capacitance on Vout to help maintain a steady Vout during a pulsed load event.  If you find that the TPS7A52 does not recover quick enough during a load transient event, you can drop in a TPS7A53 (3A) or TPS7A54 (4) with no additional effort as they are pin compatible. 

4. Yes, dropout voltage is important.  Keep in mind that dropout is a steady state parameter.  For pulsing on the load you will want more headroom to allow the LDO to perform better.  The TPS7A52/3/4 may be a better choice from this perspective.

5. I can measure the typical quiescent current for the TPS7A52 this week once I obtain an EVM.

Thanks,

Stephen

Hi Stephen

Thank you very much for your clear answer !!!

I'll wait for your quiescent current measurement for the TPS7A52.

Last question :) you write - For pulsing on the load you will want more headroom to allow the LDO to perform better

What is mean - headroom to allow the LDO to perform better?

Best regards

Ilya

Hi Wilson

Thank you, pending updates on the sample request.

Best regards

Ilya

Hello Ilya,

Headroom is defined as Vin - Vout.  Dropout voltage is a DC term, and is the minimum amount of headroom required to maintain steady state output voltage.  For transient performance you need more voltage than the dropout voltage.  The amount of voltage you need (or headroom) is highly dependent on the application and your requirements.  The best solution is to take an EVM, update it with your actual input and output capacitors, and perform a load transient on the output. You can then measure the transient response and determine if that will meet your needs.

Thanks,

Stephen

Hi Stephen

Thank you for your measurements!! But sorry, with 100uA load current the Current is 4mA - it is too high for me.

Please suggest solution that Iq will be below 50uA under 100uA load current.

Thanks

Ilya

Hi Ilya,

I took another look at our options.  The issue is that devices which are optimized for high current output (2A for example) will not be optimized for low quiescent current.  These are the best options out there for this 2A load.

Having said that, there may be another approach we can take here.  In many cases the pulsed load is allowed to droop during the peak load step, and the LDO is allowed to recharge the output capacitance during the off time of the load step.  So lets say that the LDO output current is 500mA maximum but the load pulse is 2A.  During the 2A pulse, the output will droop based on the value of Cout.  Then when the 2A peak pulse is complete, the LDO will continue to recharge the output until it reaches steady state (and before the next load pulse).  So your Vout tolerance will increase a little due to the droop during a 2A pulse, but your LDO options will increase with lower quiescent current as a result.

If you can use an LDO in this manner then we can review options with lower Iout, which should provide lower quiescent current.

Thanks,,

Stephen

 Hi Stephen

This good suggestion -> LDO output current is 500mA maximum but the load pulse of 2A will droop on the Cout.

1. I am understand that TPS7A52 is a best TI solution for output current 2A with low Iq. Please confirm.

2. Please suggest two solutions from TI portfolio - solution for my requirements with output current 600mA and 1A.

Thanks

Ilya

Hi Ilya, 

Stephen is OoO this week so I wanted to jump in here and suggest that if you need low Iq with 1A then the TPS746 would probably be a better option. Unfortunately we don't have a similar device for 2A. Below is a quick table comparing the specs of the TPS7A52 and the TPS746.