LDO INPUT CURRENT

Hi Agxin,

LDO do not have that capability - Iout = Iin + Ignd, where Ignd is the current needed by the LDO for proper operation. A buck converter does have this capability as Pin = ~Pout (minus losses due to switching, the IC, etc.), so when it steps down the voltage it steps up the current.

Regards,

David

Hello sir
I Was new to power supply component selection.Thank you so much for your valuable reply.as per the concept of LDO if an input voltage is given it will drop some voltage and provide a voltage less than the input voltage but about the current in the LDO the output current will allways be less than the input current
consider if I am connecting this LDOoutput1.5v to a load consuming 1Amps(while designing) of current the input to my ldo is from a buck converter 3.3v can be loaded up to 5A .if the requirmet of 1A from the LDO is increased to 3A Can I use the same LDO that is designed for 1A
if I can use this what are the other parameters I have to take into account(because the input side of the LDO can take up to 5A from the converter converter will provide 5A witout dropping its voltage out put.can the LDO provide 2A now )How to check this for any LDO with the data sheet parameters or through any calculation?
What are the datas that we need to collect before selecting the LDO?
(Both electrical and thermal)
e.g This are the only specification given to me
1.Vin =3.3v
2.Vout=1.5v
3.Iout=3A
thanks for helping me in advance
thanks and regards
AGXIN.J

Hi Agxin,

If the LDO is rated for 3A you can use a 3A LDO for 1A and 3A. You do need to be care of the LDO heating up in those conditions as for a 3.3V to 1.5V conversion at 3A you are dissipating 5.4W across the LDO which is beyond what most LDO's are able to thermally handle - this is where a buck converter is a better option as you avoid the thermal issues at higher currents. Some of our current options for this include the TPS744 and TPS7A7300.

For a quick calculation on thermals, you can use the RthetaJA parameter from the thermal information table to find your rise in junction temperature from ambient (Tj_rise). Tj_rise = RthetaJA*Pd, where Pd is the power dissipated across the LDO.

If you lower your input voltage to avoid thermals you need to avoid dropout, which is where the headroom (Vin-Vout) gets too low for proper operation. This minimum headroom will be specificed in the datasheet's EC table, and typically you want to have a few hundred millivolts above the maximum dropout to improve performance.


Regards,
David

Hai sir 

Thanks for your reply .In all LDO The component vendors are providing some feed back resistors are to be used in the output of the LDO to adjust the o/p voltage 

are there any ways to limit the current of an LDO.( i.e the rated current is 3A can I fixed to 2A  by some mechanism ?.or If  I have to limit the current then I have to go

for an LDO with rated outputcurrent to be 2A ,How to fix eactly that this much rated current LDO will be sufficient for my application  ?(i.e summing up all the load

currents  powered byLDO is sufficient or any additional parameter is required to be included )what can I choose an ldo with a rated output current max when my 

load current is known ? how much tolerance can I have from my Load current?.Thanks for responding again 

thanks and regards

AGXIN,J

Hi Agxin,

We have a few parts with an adjustable current limit, but typically many customers will oversize their LDO for the application by 10-20% in case their load current estimate is not correct, or they have a device that draws a high peak current during start-up or normal operation. For sizing an LDO to your application you just need to know the peak current draw by the system and make sure that you do not hit current limit during this time.

Regards,
David