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    Switch +/- 15v supply for audio preamp

    Hi ,

    i'm trying to find a low power solution to switch +/-5v on/off with daisy chained SPI to switch on/off TI PGA2500 preamp prototypes i'm designing.
    I already have SPI running to the PGA2500 IC's for other control ( phantom power switch using optomos )
    The optomos switch phantom from GPO pin controlled by SPI in the PGA's but GPO pins no use if the PGA is switched off !

    Ideally the logic / SPI would be isolated from the power rails to isolate noise as the power is ultra low noise.

    The PGA2500's use +/-40mA Maximum at 5v

    SPI controlled SPDT switch that can cope with +/-rails ?
    small relay with a SPI logic switch IC ?

    I'm trying avoid running 8 logic lines from the MC to all the preamp cards and use the SPI bus.

    Ideally the SPI SPDT thing I'm looking for could handle +/-15v rails to.
    That way I could switch on / off other line drivers and headphone amps I have in the design.

    any ideas would be really welcome :)

    - - - Updated - - -

    2nd thoughts , SPI controlled switch would not be daisy chained with PGA2500. PGA2500 is CS pin active low so some kind of SPI controlled 8 channel SPDT IC that can switch +/- rails that's active on CS line high ?
    I should also say I have 8 dual rails to switch on and off.

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    Re: Switch +/- 15v supply for audio preamp

    wait.. I just need to run a 2nd CS line from MC to select the chip. forgot how to use SPI for a min. !

    so far I can find MAX335 , a Serial Controlled, 8-Channel SPST Switch that can be daisy chained.

    from the data sheet...
    "All switches conduct equally in either direction, and on-resistance (100Ω) is constant over the analog signal range."

    using ohms law and switching 5v this is maximum 50mA load on each switch with +/-5v supply? or its 100mA since the power supply would be +/-5v ( 10v ) ?

    Is this kind of device something should be considered for power ? or its meant to be used for switching analogue signals only ?

    - - - Updated - - -

    space saving x4 optomos with resistor Panasonic AQS221FR2S is very expensive !



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    Re: Switch +/- 15v supply for audio preamp

    50 mA across 100 ohms would yield a voltage drop of 5 volts. Worse, the power dissipation, with all the channels enabled, would be 2 watts. I haven't seen the device ratings, but unless it is an IC package specifically designed with thermal enhancements, I don't think it would survive.

    The switch that you choose, whether solid state or relay, should have -I would say- a resistance in the neighborhood of 1 or 2 ohms.
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    Re: Switch +/- 15v supply for audio preamp

    I'm thinking of trying,

    MCP23S17 I/O Expander with spi Interface. 25mA per GPO.

    1 x optoMos CPC1008N for each + & - rail.

    The CPC1008N says On-Resistance of ( typical 4.8ohm max 8ohm )
    the part claims it can sustain 120mA at 80c , i'll need 38mA @ 5v

    I found CPC1008N available cheap.

    do you think that will work ?



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    Re: Switch +/- 15v supply for audio preamp

    Do you need isolation between SPI and supply switches? If not, there are simpler solutions than optoMOS.



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    Re: Switch +/- 15v supply for audio preamp

    Yes I need to isolate the digital from the preamp. I can isolate the GPO lines or SPI lines instead ?



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    Re: Switch +/- 15v supply for audio preamp

    I could send 8 logic lines from MCU though something like Si8380P for isolation.

    I'm not sure what to make the switches with since there is negative voltage.

    Can I do this with N or P channel mosfet array ?

    I don't really like the idea of switching on/off the ground to the preamp card with the N channel.
    maybe that could shut off both rails with 1 x N channel mosfet since there would be no return path to Ov?

    Otherwise 1 x P channel for each rail ? how about the negative voltage though ?



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