Multiplexing MOSFET LED Driver

Discussion in 'The Projects Forum' started by JNBUK, Nov 7, 2012.

  1. JNBUK

    Thread Starter New Member

    Jun 4, 2012
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    0
    I've designed a circuit for a project I'm working on. Its my first stab at designing my own circuit and my first time using SPICE so I'm looking for any commentary on the circuit design its self, layout in spice and anything else you feel may be relevant - I'm here to learn.

    Voltage sources 1 through 6 will be 5V PWM sources and 7 through 10 are 5v digital inputs.

    The idea is to multiplex over the 4 banks of LEDs (~80ms per bank) at a PWM frequency of ~500hz

    This allows me to control the brightness of 24 LEDs using 6 PWM pins and 4 digital.

    Thanks!
     
  2. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Your schematic is tiny and the very dark background and chicken-pox dots blocked most text. I added some contrast in my image viewer program for me to see it.

    Datasheet Archive.com has never heard about your ZVN3308A Mosfets so I couldn't see its datasheet to see if it turns on with a gate-source voltage of only 5V. Please attach its datasheet.

    Your Mosfets for V7 to V10 WILL NOT WORK since they are N-channel source-followers. When the gate goes high to +5V then the source (the output) will be 0V or barely any voltage.
    You should use P-channel Mosfets to pull the anodes of the LEDs high but then the gate signals must use level-shifters.
     
  3. JNBUK

    Thread Starter New Member

    Jun 4, 2012
    8
    0
    Here's the Datasheet for MOSFETS 1-6.
    http://www.redrok.com/MOSFET_ZVN3306A_60V_270mA_5O_Vth2.4_TO-92.pdf

    As for MOSFETS 7-10 I haven't settled on which model I'm going to use, I have a whole bunch of different types of varying suitability in my parts bin and have also been looking at driving them at 12V with an opamp from the 12v rail.

    How does that sound?
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    If you drive the gate of an N-channel Mosfet to +12V and its drain is at +12V then its source rises to only a few volts.
    Most opamps do not switch their outputs quickly. Then when they drive P-channel Mosfets they ramp instead of switch.
     
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