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9 years, 9 months ago.
Vin range for mbed LPC1768
The information on the web site indicates the Vin range is 4.5-9V but the regulator spec indicates it can handle up to 15 V. I have a 12 V supply I would like to run with this. Would that be an issue with the 10V reverse voltage limits on the BAT60A? Could I just change those parts to PMEG2020AEA?
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1 Answer
9 years, 9 months ago.
The main issue is not the reverse voltage but the maximum dissipation. The regulators can withstand the voltage but depending on the loads that your output pins have to drive there may be a thermal overload at inputvoltages above 9V. This problem is the reason that the Vin specs were changed wrt the data shown on the very early mbed documentation.
I need < 10 mA from the 3.3 V to supply what is on my board and the outputs I am driving will require < 5 mA. Assuming I wanted to risk trying to power the board at 12 V, would I need to change those two diodes?
posted by 23 Jul 2014The dissipation is calculated as (Vin - 3V3) * Imax. Note that Imax consists of the current drawn by the mbed board itself and by the output loads. That total dissipation should stay within the limit of the voltage regulator given the cooling conditions (eg forced airstream). See earlier discussions here and explanation below by given by Chris Styles.
Quote:
The main power rail on the board is 3.3v, and so any voltage applied to Vin is regulated down to this using an LDO, which dissipates power as heat. For a given load current (mbed usually draws about 150mA) you can calculate how much power has to be dissipated as heat as Vin-3.3v * 150mA.
For Vin at 9.0v and nominal load current of 150mA, this equates to 855mW, which doesn't lead to significant increase in heat.
However, at 14.0v this power dissipation is 1.6w which is more significant in terms of temperature rise. When you then add in the fact that you can be drawing up to 400mA through the IO pins, the power dissipation of the LDO can lead to thermal shutdown - the LDO switches off so it can cool off.
While datasheets and a bit of maths can show you the power dissipation and heat rise for a given load current and a supply voltage, we are suggesting that unless you are willing to experiment, and work the numbers, you should keep Vin at 9v or below.
I have successfully run my mbed at 12v knowing that I will not be drawing any power through the IO pins, and understanding that the LDO will get hotter than usual.
If you are not sure, keep it below 9v.