victoroza
Member
Hi,
Maple Mini R3 has silkscreen that says max. 5V for Vin. I assume that's incorrect and the max. Vin voltage is 12V. Also, the Vin must be greater than 5V in order for Maple to be powered by external and not USB power.
Please let me know if my assumptions are correct.
Thank you.
gbulmer
Moderator
victoroza - The Maple Mini's STM32F103 is intended to operate at 3.3V (or less). So the question is about the voltage regulators which are on the Maple Mini.
The voltage regulator part on the Maple Mini schematic is MCP1703T-3302E/CB:
http://www.microchip.com/wwwproducts/Devices.aspx?dDocName=en530838
The datasheet says it has a dropout voltage of 0.625V, when the input voltage is 5V and supplying 250mA. So in theory it could supply 3.3V at a Vin of 4V.
However, the Maple mini schematic shows there are diodes between USB power and another between the Vin pin, and the regulator. The schematic doesn't say what part is used, so lets assume a simple diode, with a 0.7V forward voltage drop (it might be a Schottky diode with 0.4V, that's what I would use). So Vin would need to be 4.7V (or 4.4V with a Schottky diode). USB power and Vin diodes are in parallel, so yes, the voltage on Vin would have to be slightly higher than 5V if the power is to be drawn from Vin. Vin could be lower, and USB would 'win' when its plugged in, and then power would revert to Vin when USB is unplugged.
The datasheet for the MCP1703 says it can tolerate an absolute maximum of 18V, but this is absolute maximum under optimal conditions, so we need to do a calculation or two before assuming that is in any way feasible.
The voltage drop (at whatever current is being used) between Vin and 3.3V is converted to heat by the MCP1703. So we need to calculate the power being dissipated to get an idea about the actual safe input voltage. If you look through forum threads you'll find a bunch about people who have tried higher voltages, and burned out the regulators.
Lets assume the STM32F103 is not driving anything significant (no LEDs, nothing). The STM32F103xB datasheet says the maximum current at 72MHz, running all peripherals, but not driving any loads is 50mA. (There are actually two voltage regulators on the Maple Mini, the second one supplies power to the analogue section of the chip, but it consumes so little power, I'll ignore it).
The MCP1703 datasheet says the internal regulator (the piece of silicon) can operate at 125C max. Let's assume the surounding air is 25C (to make the arithmetic easy). The thermal resistance from Junction to air is 336C/W, i.e. temperature will rise by 336 degrees C if it is trying to dissipate 1W (this is conservative as the Maple Mini PCB will dissipate heat, and improve the figure by making it a smaller temperature rise/Watt, but lets just bound the problem).
So a 100C rise would be 100/336 = 0.297W (call it 0.3W for easy arithmetic)
Power = Voltage * Current
rearranging
Voltage = Power /Current
we know current could be 50mA, or 0.05, and Power is 0.3W, so
Voltage (across voltage regulator) = 0.3W / 0.05 = 6V is okay for an STM32F103, running all peripherals, but with no load (the on-board blue LED should take less than 1mA, there is a 1k current limiting resistor, so I'll ignore that.)
Remember there is a diode in the circuit too, dropping 0.7V, or maybe 0.4V (and converting that power to heat too), so the actual voltage applied at Vin could be 0.4 to 0.7V higher.
Vin = 3.3V + 6V + 0.7V (0.4V for Schottky diode) = 10V max
running *ONLY* the STM103 and the user LED, at an ambient temperature of 25C
Lets consider 10 LEDs using 5mA each (modest), pushing the current up to 100mA, 0.1A, and hence
Voltage (across voltage regulator) = 0.3/0.1 = 3V
Vin = 3.3V + 3V + 0.7V (0.4V for Schottky diode) = 7V max running the STM103 and 10 LEDs (5mA each)
Say the Maple mini were sitting in a slightly enclosed space, it's a warm day, and the ambient temperature on the PCB rises to 50C
Power dissipation for the MCP1703T with a maximum of 75C rise becomes
75/336 = 0.223W (call it 0.22W)
Say the Maple is running at its maximum of 150mA
Voltage = 0.22W/0.15A = 1.47V (say 1.5V)
Vin = 3.3V + 1.5V + 0.7V (0.4V for Schottky diode) = 5.5V max
There is a further problem. As the Maple Mini's PCB warms, the ambient temperature around the voltage regulator increases. This reduces the rate it dissipates heat, so its safe temperature-rise is reduced, so the safe power through it reduces too.
Summary:
1. These are conservative calculation, but it is based on manufacturers published data.
2. 12V on Vin is unlikely work for very long
If you want to use a 12V supply, I'd recommend using either a 5V regulator, with good power dissipation, or a DC-DC converter, and feed that into Vin.
(Full disclosure: I am not a member of LeafLabs staff.)
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