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Power managament PCB design

MadhuWesly , 10-06-2017, 04:19 AM
Hello all,

After I have gone through the following data-sheet, I came across few doubts, can you please give your explanations in lines to them:

Output requirements: 3.5V, 2A and 3.3V, 1A.
Input voltage around 7.5V

Ambient temp: -40 to +60 degrees

Given in data-sheet:
Junction temp: 150 max
Junction to Ambient temp: 34 degrees/watt

1. From above values, can you please tell me will there be any over heating issues? What is the formula for calculating Max. Dissipated Power in SMPs?

2. I want to implement the same application circuitry provided in the data-sheet, Will it work? (consider I will changing Voltage divider resistors values, and Inductors accordingly to achieve desired outputs)

3. The output ripple given in an example is 30mV, will it effect more?

Awaiting your reply.

Thanks & Regards,
mairomaster , 10-06-2017, 05:59 AM
If your output power is 10W lets say and your efficiency is 90% (don't have time to check the datasheet now for exact numbers), the dissipated power will be roughly 1 W. That will cause 34 degrees of junction temperature increase compared to the air temperature, which should be safe.

If you consume twice less power, then you will dissipate about twice less as well (not exactly since the efficiency will be slightly different).
robertferanec , 10-06-2017, 07:04 PM
@mairomaster, awesome answer!

1. usually, your power supply max current should be able to cover peak currents which are higher than average application currents, this means, when you say 10W, it will not probably run on 10W, so the heat may be even lower. Also, from my experience, sometimes inductors may get hotter than transistors.

2. It should, I often follow datasheet and just update resistors, inductors and capacitors as recommended in datasheet. Always works ok

3. I do not know

If you are not sure, buy evaluation board, test it measure it (do not forget, size and number of planes will influence temperature) ... it's cheap and you will be sure (i do not know, maybe they have this one?): http://www.ti.com/tool/tps54295evm-057
Luca , 10-16-2017, 11:01 AM
The output ripple depends by a lot of factor, the main key to consider is:

1 - The amount of current ripple that you design the inductor. Tipically 20% to 40% of average output current is the choice for this kind of SMPS. Remember, more ripple in inductor will mean more output caps (also with less ESR) for achive the same voltage ripple, but better performance for current loop regulation feedback (this kind of SMPS are almost alway controlled in current mode).

2 - Value in uF, ESR and ESL of output capacitor. With high frequency, an high current, SMPS (like this) you need to take in account the effects of PCB, i mean PCB resistance and inductance.
Consider only the ESR and ESL of output caps it's not sufficient.
Take in mind that ceramic caps suffer the DC bias...

3 - Attenuation factor of output voltage resistor divider and "point" where the output voltage is "sensed". I mean that you will have different voltage ripple if you sense the output voltage at the outputs caps, far from the load, near the load.. and so on.

In generally SMPS is "simple" (not a lot of tracks like BGA or stuffs like that) but there are very critical on PCB layout (especially with high frequency and high current), component choosed (not only the value but also the "type") and, if is possible, i plan how and where place the components taking in mind that SMPS is "the heart" of that components and i start the layout with the best placement of SMPS components and than other.

I hope that this will help you, we are here!
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