what diode should i use for the LM4780 mute function?

[TiGeRGuY]

hi, right now i am designing a pcb for a paralleld LM4780 for each channel.
i'm planing on using the mute function for a softstart.
i want the capacitor to discharge fast, so the softstart will occure the next time i turn on the amp even if i do it right away.
so.... this is why i'm gonna have to short the capacitor and the V- (-30V)
and have a diode there.



the thing is i know nothing about diodes and i am asking for the type of diode i have to use, and its values.

tnks for the help!

[TiGeRGuY]

guys this is a pretty simple question compared to the discussions here.. no one has an answer? :\

[Speedskater]

Do you mean the 1N4004 diode?

[TiGeRGuY]

yeah, this image captured from my schematic in eagle, so this is the diode package i put temporarily.
as i mentioned, right now i am designing the PCB so i realy need to know what diode to use and it's physical size.

[djoffe]

The 1n4004 will be fine...you need some a little beefy to handle the current from the 100 uF...the 1n4004 will be fine.

However...75K plus 13k7 is too high for a 30 V negative supply rail. You won't really get the 0.5 mA the data sheet calls for...

You need the sum of these resistors (assuming you're driving a single mute pin) to be less than (30-2.6)/0.5mA=54.8K...Maybe a total of 40K would give you a safe margin...that will change your mute time constants, but at least you'll come all the way out of mute...

[TiGeRGuY]

hi djoffe, tnks for your comment!
the RM2 resistor is calculated for a current of 1mA for each mute pin as suggested in the datasheet (this is a LM4780 so it has 2 mute pins).

the calculation:

RM2=(30-2.6)/2mA=13.7k

the RM1 is calculated to have a 4 sec delay when the AMP starts. (i'm having another circuit with a relay that close 3 sec after the AMP starts and i don't want the closing sound heard from the speakers)

does it still look wrong?

[Redshift187]

The current has to flow through RM1 and RM2. Once CM is charged, you can just sum the values of RM1 and RM2. Calculate the values without the capacitor, then change the value of the capacitor to change the delay. Also, it states in the documentation that you should double the minimum required current to 2ma for both channels of a LM4780.

[TiGeRGuY]

if i understood you correctly RM1+RM2=13.7k so the current will be 2mA (1mA per mute pin).

lets say RM2=3.7k and RM1=10k

according to my calculation T=7.5 (t=4sec)
T=R*C
C=T/R =7.5/10k = 0.00075F = 750uF

now i just have to make the capacitor a 1000uF and the resistor 7.5k

then RM2=13.7k-7.5k=6.2k ?

is this what you ment??

tnks for helping by the way!

[djoffe]

It's a good idea to post your schematic...that way, you can avoid the factor of 2 errors and other problems that come from us making assumptions about what your are doing...assumptions that might not be true...Bvadai!

[TiGeRGuY]

http://img39.imageshack.us/img39/5455/pa100scheme.png