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Understanding the Netduino 2 schematics - LMV7271MF and NTR4101PT1G
I'm building my own PCB for the Netduino Mini, and I'm exploring the possibility to have the same power-handling functionality that the Netduino 2 has, that it doesn't really matter if you power it via the powerbarrel or the USB.
When looking at the schematics for Netduino 2 I believe this is the purpose the LMV7271MF chip. Am I correct?
So the LMV7271MF seems to sense if either the powerbarrel (the VIN_PROTECTED) or the USB 5V is plugged in, and somehow that decides which powersource to use. Right?
What is the purpose of the NTR410 MOSFET? Is it the source side of the MOSFET that is used as input to the 5V-to-3V3 regulator?
The basic operation of the circuit is to switch the USB power on, when voltage from power jack drops below a certain threshold.
The LMV7271 operational amplifier (op-amp) is used as a comparator, which produces output voltage VO = "1" if V+ > V-, and VO = "0" if V+ < V-. Thus, when VIN_PROTECTED < VIN_REF (*), the op-amp output switches the MOSFET on (**), which connects the USB power to +5V rail. Otherwise, USB power is disconnected and +5V rail voltage comes from 5V linear regulator U1 (MC33269).
(*) The exact voltage levels being compared are determined by resistor dividers:
so the comparator output voltage VO = (VIN_PROTECTED/2.4) > 2.5V ? "1" : "0", which means the USB power is switched on when the power jack voltage drops below 2.5*2.4 = 6V. This is because the MC33269 regulator has 1.0V dropout voltage (its input voltage must be higher than output voltage, to ensure proper function).
(**) It is a P-channel MOSFET, which has inverted switching logic - it is "on" when the gate is "low".
As CW2 mentioned, the opamp is there to switch between USB and VIN power automatically (so you don't have to worry about damaging the board by plugging in both power sources at the same time).
This is great for development boards--but probably not necessary if you're building a tiny NETMF device.
Wow, good answer CW2, I understand what it does now
Chris, you are probably right - it's a nice feature to have when you deploy and test, that you don't have to have battery connected at the same time, but not really a must-have.
But, as you say, it's there to avoid damaging the board when plugging in both USB and power jack at the same time. What would happen if the protective circuit wasn't there. So both USB and 5V regulator would try to put 5Volts onto the rail at the same time. Since they have common GND, would it really matter? Or would perhaps the 5V regulator be damaged by receiving 5V onto its output?
I'm going to create a prototype PCB the coming days where I'm testing quite a few new chips and circuits, so I'll include the circuit there, but but probably drop it from the final PCB.
Voltage is the difference in electrical pressure between two points. When two voltages are supplied to a single point, the second power supply effectively becomes a negative voltage--out of spec and potentially damaging to your device, power regulators, etc.
One possible solution to this dilemma is to put reverse protection diodes in-line on both power supplies--a relatively cheap solution that has the side-effect of reducing your power supply's effective voltage. Another solution is to use an opamp/fet circuit to enable secondary power when primary power is lots; that's a more expensive solution (which we used on Netduino) that doesn't waste a lot of your power supply's available wattage.
The reverse protection diodes will cause, what, a 0,7V drop? That is the same as the SS15 schottky diode you have near the power jack, right?
So if I put an SS15 between VBUS on the USB, and the 5V input on the Netduino Mini, I assume I'll get a 4.3V supply to the Mini when I power it from the USB cable. Is that enough for the Mini?
The micros run at 3.3V. So you'll typically put an LDO or other power regulator between your higher voltage (4.3V/5.0V/9.0V/etc.) and the target voltage. A 3.3V LDO will have a dropout voltage--so the input to the LDO will need to be at least as high as 3.3V + dropout_voltage.
In the case of USB, its actual voltage will be in the 4.75V-5.25V range (and a bit wider than that with some USB power supplies and with USB 3.0 devices). If you're running a light load, the dropout voltage on an LDO will typically be pretty low (much lower than the "maximum dropout"). LDO datasheets will often include a graph demonstrating the dropout at various current draws.
Yes, those diodes are reverse voltage diodes. But they're used for another purpose...protecting the power barrel against "center negative" power supplies. Most power supplies give positive voltage to the center pin--but we wanted Netduino to be simple to use so we added extra protection there against reversed DC adapter connections.
Chris, so the ARM7 on the Netduino Mini takes in 3.3V, and it is supplied by the LDO Converter MC33269D-3.3. It has a max dropout voltage of 1.25V, so it seems to me that I have to supply at least 4.55 volts (worst case) onto the 5V rail of the Netduino Mini. In a typical scenario the dropout voltage on the LDO is about 1V, so it could work with as low as 4.3V.
So it seems like putting a schottky diode inline with the VBUS from the USB might cause the voltage to swing from 4.05V (4.75V-0.7) to 4.55v (5.25V-0.7).
It would be pretty much on or under the minimum input-voltage for the Netduino Mini board. Hmm, seems like the inline schottky diode isn't a viable idea. Seems like when I want to power the Netduino Mini from USB I'll need all the voltage the USB has to offer.
If you don't need 800mA max. output, but rather 500mA. This is a nice LDO part with 0.5V dropout at full current output. http://www.micrel.com/_PDF/mic5219.pdf
To add to what baxter said, check the drop value in the LDO datasheet for the _total wattage_ that you're consuming. LDOs typically list their maximum dropout in the headlines--but the actual "minimum dropout" is usually much less when you're not using the maximum-rated-current with the LDO.
And as baxter mentioned, an 800mA LDO is pretty overkill for a tiny project that doesn't use much power. We often use tiny LDOs which are rated at around 100mA for commercial products.
Thanks, both the MIC5219 and the LTC4411 look very interresting!
Looking at the LTC4411 it seems to be a good candidate for switching between power from USB and battery.
The MIC5219 is also very interresting, but let me just clarify that I'm using the Netduino Mini, and my calculations above were regarding the LDO that is already on the Mini. When powering it from the USB I'm not going to add an LDO between the USB and the Mini.
But back to the LTC4411. I can put the LTC4411 between the USB VBUS and the 5V input of the Mini. Then, my 11.1V LiPo battery (which is also supplying two motordrivers) can be connected to a LM1117 I have already ordered for 5v regulation, and from there used in the same way as the "Wall socket input" is in the LTC4411 link baxter put in. Not sure if i need a schottky in that scenario, or if it should be put before or after the LM1117.
That way I might end up with a circuit which allows me to power the Mini from both USB and battery, and hopefully safely switching between the two powersources.
Now when you mention LiPo battery - do you plan to have charging functionality on board? Because there are Li-Ion/Li-Po battery charge controlers with automatic power source selection. For example, Microchip Application Note 1149 shows a design with MCP73837 in which "The input power should supply the system load and charge the battery when a battery is present in the system. When the input power source is removed, the system is supported by the battery. When the system load and the battery draw more energy than the supply can offer, the system load takes priority over the battery charger."
So, you can for example power Netduino Mini always from the battery and let the MCP automatically handle the power source selection and battery charging. You'd probably need to select a little bit different battery setup (a separate cell for Netduino Mini?) or choose multi-cell charging IC (MCP73837 is only for a single cell, 4.2 - 4.5V).
For this project I think I'll keep it simple and charge my LiPo the "good-old-way", but I'll definitely look into the MCP73837 chip if I create a system that should be rechargeable in the future.
