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19th November 2019, 08:01 #1
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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 :)
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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.
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19th November 2019, 12:17 #2
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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 ?
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space saving x4 optomos with resistor Panasonic AQS221FR2S is very expensive !
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19th November 2019, 16:13 #3
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.My batteries are recharged by "Helpful Post" ratings.
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19th November 2019, 18:38 #4
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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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20th November 2019, 00:22 #5
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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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20th November 2019, 03:10 #6
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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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20th November 2019, 09:14 #7
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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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