Dac Power Supply

[kkchunghk]

Hi,
I am doing a dac with DIR1703 + 2 x WM8740. So I have a 30VA transformer providing 3 parallel regulated 5V power supplies, one to each chip. After the usual reservoir cap, the regulation is done by 3 TL431s as shown in the schematic. All supplies measured nicely at 5V when there is no load. BUt when they connect to the chip, the supply to DIR1703 dropped to 2V . Connect the DIR1703 alone is fine. Am I missing something here?
Cheers,
KK

[Black Heart]

I don't think the DIR1703 is a 5V chip, is it? But it is hard to tell what your problem is without more info. What is the DC voltage to the regulators? Why the large value series resistors? Is it because you have too much input voltage? The series resistors are probably dropping about 10 volts or so, and the pass transistor some more, so it would need to be around 20VDC, right?

[kkchunghk]

Hi Black Heart,
You are right about 1703. There is a LE33Z before the chip. I have 17VDC going to the 431s. The first version was without the series resistor. One of the 431s got very hot. So I looked up some dac eval board schematics. Nearly all have this series resistor. The values are between 100R and 300R. After these, the 431s stay cool.
I take you hint that there is not enough juice from the transformer. I'll try a higher rated one and see.
Cheers,
KK

[Black Heart]

Originally posted by kkchunghk
Hi Black Heart,
You are right about 1703. There is a LE33Z before the chip. I have 17VDC going to the 431s. The first version was without the series resistor. One of the 431s got very hot. So I looked up some dac eval board schematics. Nearly all have this series resistor. The values are between 100R and 300R. After these, the 431s stay cool.
I take you hint that there is not enough juice from the transformer. I'll try a higher rated one and see.
Cheers,
KK

I don't know what a LE33Z is but that's OK. Yes, if you had to drop 12 volts or so across your pass transistors at 30 - 40 mA then you would probably get hot without a heat sink since that is around 1/2 W. Using the resistors to reduce the voltage is OK (though they do somewhat limit the performance of the regulators) but the values are probably just too high. Try going down to 200 or 100. No need for more voltage from the transformer. If anything, less voltage would be better so you could remove the series resistors (or at least reduce their value since they do have a big impact on the output impedance of the regulator). Or use transistors that can dissipate more power, or heat sinks on them.