Getting +5v from a +15v line

[bigswole]

can this be accomplished with an lm7805 without it getting to hot?  I am wanting to use a +15v/-15v psu,  and get a +5v line without having to buy another transformer.  This is to power ti pcm4222evm module.

 

[Rochey]

depends on the current consumption.

don't forget that the PCM4222 analog rail is 4V, not 5.

PCM4222's 4V rail requires 75mA max. So you need to drop 11V at 75mA, giving roughly 825mW of power dissipation in your regulator.

Now study the regulator you want, and look for the package Theta-J. That'll tell you how many degrees the package will heat up, per watt that you dissipate.

cheers

/R

 

[bigswole]

thanks rochey,  but the voltage input for the evm module says +5v in,  am i missing something?  Also do i need 2 separate grounds for the digital and analog lines?

 

[Rochey]

my bad, the 5V is used to generate the 3V3 digital.
The 15V is put through an LM317 to get to the 4V.

What are your plans for clocking it? will it be the constant master in your system?

Just to make sure you know: The BNC connector on the EVM is actually for the master clock, not wordclock.

 

[bigswole]

I do plan to use it as the constant master clock for my system.  Any other suggestions?

 

[Rochey]

It'd be good to understand what voltage = full scale, so you can set the right nominal levels for the device.

Each analog input is buffered by an input circuit employing the OPA1632 fully differential audio amplifier,
selected for its low noise and distortion, and fully-differential input-to-output architecture. The buffer circuit
provides attenuation (nominal gain = 0.482, or –6.34dB) and low-pass antialiasing filter functions. The
printed circuit board (PCB) layout supports limited board stuff options for experimentation. The full-scale
input voltage for the input buffer circuit is approximately 4.2VRMS (or +14.6845dBu) differential, given a
nominal VCC supply of +4.0V for the PCM4222 and a source impedance of 40Ω. Due to the full-scale
input voltage varying slightly from one PCM4222 device to another, the full-scale input swing should be
calibrated for each EVM individually, adjusting the input voltage level until a 0dBFS output level is
indicated.

 

[RuudNL]

If the dissipation in the 7805 (or whatever) gets too high, you could add a resistor between the input and the 7805 and split the total dissipation. (1/2 * P in resistor, 1/2 * P in 7805) Aim for a voltage drop of 5 V. over the resistor.
I have seen this more often in commercial equipment.