Suitable JFET?

[ssashton]

Hi everyone,

I have an audio amplifier IC that has a mute feature I want to control to reduce pops on power on / off.

The mute is a CMOS compatible pin where HIGH disables the mute, effectively enabling the amp.

I have found that the pin is actually fed internally in the amp IC via a high resistance so that if left open it will enable itself. When I connect a capacitor between the mute pin and ground it will slowly charge and enable the amp gently at power on. That's nice.

What I want to do is have a JFET that bleeds the pin to ground quickly when the power is switched off and the rail starts to fall.

I've only got positive voltage (+15V) available so I guess it needs to be a P-channel JFET.

I 'm thinking if I drive the gate via a resistive diver that only just exceeds pinch-off voltage when the rail is a full potential, it should close the circuit between mute and ground as soon as the main rail starts to drop. Does that sound sensible?

What kind of JFET part number would be suitable? A small SMD part is great.

[Kleinstein]

There is only a limited choice of P channel JFETs.  Chances are the MMBFJ177 or MMBFJ270 should be OK.

[David Hess]

I think practically any p-channel JFET will be suitable, but normally a chopper type would be used.

The On Semiconductor SMMBFJ177LT1G is suitable and the lowest cost.

https://www.mouser.com/Semiconductors/Discrete-Semiconductors/Transistors/JFET/_/N-ax1rsZ1yzvvqx?P=1yuk7vzZ1z0y3dt&Ns=Pricing|0

[ssashton]

Thank you!

Is there anything wrong with my concept, or a more sensible / simple way to bleed down the pin when the main rail drops?

Now noticing p-channel JFET are not usually cheap parts!

Maybe I can pull the mute pin to ground via a suitable resistor, and then drive it high only when the main rail (+15V) is over 13V-14V.

[SilverSolder]

Have you tried putting a scope on the amp, one probe on power and the other on output, to see when the pop happens compared to the power voltage?

The pop may happen very quickly...  before the mute circuit could react?

[ssashton]

The spec sheet says it can operate down to +8V so I think it will give enough time while the +15V rail drops for a mute circuit to trigger.

[SilverSolder]

Seems a reasonable assumption - I guess I might be getting paranoid in my old age, I no longer trust my assumptions without measurements to back them up! 

[David Hess]

Is there anything wrong with my concept, or a more sensible / simple way to bleed down the pin when the main rail drops?

There are other ways to do it, for instance with only bipolar transistors, but they are more complicated.

Now noticing p-channel JFET are not usually cheap parts!

Unfortunately they are not used enough to gain a lot from economy of scale of production.

Maybe I can pull the mute pin to ground via a suitable resistor, and then drive it high only when the main rail (+15V) is over 13V-14V.

I think that depends on how quickly you want to pull it to ground when power is lost.

[ssashton]

Thanks all. I think I need to play with it and see how quickly the pin bleeds down and how long the amp runs before it pops.