Switch +/- 15v supply for audio preamp

[benbiles]

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

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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.

 

[benbiles]

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 !

 

[schmitt trigger]

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.

 

[benbiles]

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 ?

 

[FvM]

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

 

[benbiles]

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

 

[benbiles]

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 ?