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F5529 LaunchPad

William Cerny
Prodigy 10 points

I am using a F5529 LaunchPad to read 3 EP4 optical encoders that require +5VDC, Will the input handle  5V or does it need to be reduced to 3.3 or 3.6?

  • 0
    Guru 58965 points
    You could simply put a resister (a few K ohm) between the 5V signal and the input pin of F5529 to limit the current.
  • 0
    Guru 12050 points

    The msp have protection diodes and will channel away voltages over 3.6V (Vcc+0.3V)
    But the diodes are limited to 2mA, so a 2K minimum series resistor is needed

    I would use 100K (unless it's high-speed signal) as the excess voltage needs to be used up and as most LDO's (the one you use for MSP's Dvcc)
    can not sink overvoltage's, so if the system is very energy efficient you could end up raising Dvcc to 5V and create permanent damage.

  • 0 in reply to Tony Philipsson
    Guru 227245 points
    Well, not 5V, as with rising VCC the current drops and will cease at latest with the clamp diode voltage. But even 4-4.7V will be possibly disastrous if applied for a longer time (MSPs are tough, and some I had survived permanent 4V supply).
    For high-speed signals, a voltage divider can be used (series resistor followed with pull-down resistor, so the excess current is routed to GND and not VCC)
    But in general, you're right. Many people have powered their MSP through a port pin (supply off, but 3V applied to a pin) and wondered why it started. The clamp current needs to be lower than the total current draw from VCC. While most LDOs have a reverse diode to protect the LDO from output voltage applied while the input is off, this will only conduct if the output is above the input. Which won't be the case if the LDO is powered by the same 5V as the encoder (or powered at all).

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