Vin, Vout range

05 Jun 2012

Is is possible to change Vin and Vout range in software? This would avoid needing more board space for op-amps in my application. Can it be done?? Tnx...

Gary B

05 Jun 2012

The mbed schematic tells you that Vin is connected to LD1117S33 voltage regulators. The input voltage should be between 4V5 and 9V or you will damage these devices and possibly the LPC1768.

Vout is the output of one of the LD1117S33 voltage regulators. It is set at 3V3 and can not be changed. You need external hardware to modify Vin or Vout ranges. The only option for Vout is to replace the LD1117 on the mbed circuitboard with another type.

07 Jun 2012


Just to be clear, the on-board regulators are there to prevent damage to any of the on board components by providing a clean, stable power supply, and their output is indeed fixed at 3.3v

The bottom end of the Vin range is 4.5v and this value is determined by the LD1117S33. We set it to be 4.5v because the regulator output is 3.3v and the regulators *worst case* drop out is 1.2v. You might find that you can still power your mbed below that.

The top end of the range is set by heat dissipation. At 9v, the regulator is dropping 5.7v. As the mbed could be drawing anything up to 220mA (with LPC1768 and interface devices active, current being drawn from GPIO pins, the Ethernet Phy and oscillator active, etc) the heat dissipated is 5.7v * 0.22A = 1.25w

There are many scenarios when it will be perfectly safe to go above 9v on Vin, but if you do, you need to consider the heat issues first.

Hope this helps!

07 Jun 2012

oops, I've not been clear... the question I have regards the adc and dac in/out ranges. can the adc vin and dac vout ranges be altered (preferably via coding or possibly a rewire) ?? tnx guys...


08 Jun 2012


The answer is still no though, unless you start doing board mods.

The Analog supplies and voltage references are are connected to the same 3v3 supply (with a little filtering and such). On other LPC1768 platforms, for example the Keil MCB1760 you do the ability to make your own provisions for the Analog supplies and references.

So yes, the LPC1768 does support that behaviour, but for simplicity, we've not exposed it.

Hope this helps.

Thanks, Chris

13 Jun 2012

I am using external op amps to change the Vin (at amp) to +/-12V and similarly the DAC out to the same range. In doing this I use the 3V3 supply pin as the bias for the bipolar to unipolar conversion, but was concerned this was from a different 3v3 regulator used by the onboard mbed analog reference. It would have been useful if the onboard 3v3 analog reference was available externally for this purpose. Similarly, I believe the noise on the DAC output could be reduced if the analog 0V was made available for the analogue interfacing requirements.

For the A/D input I only require 1 measurement per second, but I perform 25 measurements 10uS apart and take the average, by averaging the noise is reduced and provides sub-bit resolution.