Power On-Off Sequence

[Quercus]

Hello all,

I'm working on an audio amplifier based on the TDA7294 amplifier (**broken link removed**). The component has both a mute and standby circuit and the recommended sequence for power on-off is: Off - Power On - Standby Off - Mute Off - Play - Mute On - Standby On - Power Off - Off. Note that turning standby off and mute off means setting the associated pin to high, to run them on brings the pin to low.

I'm looking for a circuit that will execute the three tasks in order when the power switch/button is pushed, and then again in the reverse order when it is turned off. As far as switching the 120V input to the power supply... I don't care if the sequence circuit uses a mechanical switch, relay or solid state switch.

Any suggestions are appreciated.
Quercus

 

[Diver300]

I can think of two possible solutions. One is to use a microcontroller to run the sequence. That could be a very simple device, or as part of a more complicated function. You should make sure that there are pull-up resistors that turn the amplifier to mute and standby while the microcontroller is turning on.

My other solution is to have a capacitor charged up slowly through a resistor when the amplifier is wanted to be on. Then have comparators that compare the capacitor voltage with fixed references, and the comparators turn on the mute and standby. By having the fixed reference for the mute at a higher voltage than the standby, it would always turn the mute off after the standby was off. When turning off, the capacitor would be discharged slowly, and the mute would turn on at a higher voltage, so sooner, than the standby turns on.

Either of these solutions could include power switching as well, with another microcontroller output or another comparator from the same capacitor and a different voltage reference.

 

[crutschow]

Below is the simulation of a power-on sequence circuit that uses a CD4015 4-bit shift-register and a CD4049 inverter.
The power switch is simulated by generator V1 controlling switch S1, pulsing at 1 pps.
As you can see it continuously sequences through the power on sequence and then off in the reverse order. (The green and red traces are vertically offset to make them more visible. The are all the same amplitude.)
D1, R2, and C2 form a switch debounce circuit.
C1 and R1 are a power-on circuit to reset all outputs to the zero state at power on.
Vcc can be any DC power from 3V to 15V.
The Pwr_On output from pin 5 can be used to drive a mechanical or solid-state relay to control the 120Vac. This will require a buffer transistor to drive the relay. If you need advice on that, I can provide some.

Note that this circuit requires power independent of the 120Vac amp power so you may need a small wall-wort supply for that.

Not that all unused inputs on both packages must be connected to ground (not shown) or the circuit may become erratic.

 

[Diver300]

I don't think that is the sequence that the OP wanted. He was looking for first on, last off. The shift register gives first on, first off.

 

[crutschow]

I don't think that is the sequence that the OP wanted. He was looking for first on, last off. The shift register gives first on, first off.

You are right, of course.
Back to the drawing board.

 

[Quercus]

Thank you all for the thoughts - I'll start looking into the capacitor idea unless there are other thoughts.

Crutschow... I like your quote... 'inside every little problem...' only too true.

 

[crutschow]

Here's an LTspice simulation of the analog sequence circuit as suggested by Diver300.
The LM339 has four comparators in a package so you just need one package for those.
S1 and V2 simulate the power on and off switch.

Note that a source for V1 is required that's independent of the main power that's switched by the Pwr_On signal.
This can be provided a small wall-wort.

 

[Quercus]

Thanks, Crutschow. I appreciate the input.

 

[tomizett]

I don't want to complicate this too far, but I thought the folowing was worth a mention.
In comercial amplifiers, it is normal for this kind of power-sequencing circuirary to be driven from the main power supply in such a way that it keeps the amp muted for a while after switch-on to allow the power supply to stabalise. Then, when power is removed, it must mute the amplifier BEFORE the amp's power rails drop too far.

Typically, this might be implemented by a circuit like crutschow shows in #7, but driven from a sepperate rectifier/smoother circuit from the main amplifier supply. You make the smoothing cap small and load it relatively heaviliy with a resistor, so that the voltage in C1 (in the above diagram) always drops below the mute threshold before the amplifier starts to misbehave due to having too low a supply voltage.
Just a thought!