Noobie problem driving multiplexed LED array with P-channel mosfet

"CJ"

** Never thought of placing a diode between the uC and the base of each PNP ?? ** That number is for an N channel fet.

.... Phil

NP

On the diode, I thought about it, but the transistor is a PNP driving the high side, and it's 4.2V in vs 3.3V on the uC, so in order to turn it off I would need to pull it high with a resistor to the high side supply... but then when I try to drive it low with the uC, the diode is going to prevent that, isn't it?

I made a mistake on the part number, the PN of the MOSFET is AO3415 (Alpha Omega Semi).

"CJ" "Phil Allison"

On the diode, I thought about it,

** Shame you did not TRY it instead of inventing false theories.!!

but the transistor is a PNP driving the high side, and it's 4.2V in vs 3.3V on the uC, so in order to turn it off I would need to pull it high with a resistor to the high side supply...

** Nonsense.

but then when I try to drive it low with the uC, the diode is going to prevent that, isn't it?

** Draw the circuit - imbecile.

I made a mistake on the part number, the PN of the MOSFET is AO3415 (Alpha Omega Semi).

** Bloody BIG mistake - wrong brand and wrong number entirely.

The GS threshold voltage is catching you out - at 0.9 volts, Id is up to almost 1 amp.

Do read the data.

..... Phil

PNP

Even with a pull-up, you might exceed the absmax of the uP. and that is assuming there is no protection diode on the uP to the + rail. Why don't you just buy a driver chip? MAX7219 for example.

Even if you get the voltages figured out, all that external crap will probably lead to ghosting as you multiplex.

Install the two diodes in series with cathodes toward the uC - this will allow the PNP base to go up to ~ 4.7 volts when the uC output is high. A pull-up resistor on the base will ensure that the PNP turns off. You should probably also have a resistor in series with the diodes.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
GPS and NMEA info: http://vancouver-webpages.com/peter
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You need to get the gate voltage to within a couple hundred mV of the source to make sure it's turned off completely (but need a couple of volts to make sure it is turned on). That spells a bunch of additional parts.

Why don't you use a complimentary PNP/NPN pair with internal resistors in SOT-23-6 or smaller (reversed logic)? No external parts... assuming your current is low enough for those parts to handle it.

That should NEVER happen if the firmware is written properly.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

y d t

ach PNP

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"The Journey is the reward"

eff.com

Regarding ghosting, you add a dead cycle in the muxing, which is what the MAX7219 does. However, this guy is a self-professed newbie.

Peter Bennett a écrit :

Just use a common gate NMOS as a level shifter.

------+---------+-------- | | .-. | | | | | | | '-' | | | +------||-+ | ||->

||-+ ||-+ ||

--
If you can drive the shift register with the µC 3.3V I/O's, this should
work: (View in Courier)


5v>---------------+---------+----------------+
                  |         |                |
                 [R]*       |                |
                  |   *     E                |
3.3V>-+           +--[R]--B PNP/PCH          |
      |           |         C                |
    +---+   *     C         |             +--+--+
    |µC |--[R]--B NPN*      +---[LED1>]---|     |
    |   |         E         .             .     .
    |   |         |         .             .     .
    |   |         |         .             .     .
    |   |         |         +---[LEDn>]---|     |
    |   |         |                       |     |
    |   |===============/CONTROL/========>|     |
    |   |         |                       +--+--+
    +-+-+         |                          |
      |           |                          |
GND>--+-----------+--------------------------+

* Additional parts per channel

JF