ProtoDAC TDA1387 X8 project

[hifinet]

I should mention that Gaby's low noise LM317 regulator will work with either AC (use a transformer for a linear supply) or DC input. The black part between the input terminals and the 50V electrolytic capacitor is a full wave bridge rectifier, which allows AC input.

[hifinet]

Do you mean getting the 5V from the pin 2 and/or 4 of the RPi GPIO? I don't think the voltage differential will be enough. The voltages are usually less than 5.1V, and that is right on the margin of the dropout voltage of the LT1963 to get 5.0V with the current draw of ProtoDAC. You will need at least a 100mV differential. If the voltage differential is less than indicated in the chart on page 1 of the datasheet, the LT1963 won't regulate.

Thanks, so I can get 5v from pin 2/4, but LT1963 will most likely not work. Understand.

The way it could be done is to change one of the resistors on the board. See the adjustable regulator Figure 2 on page 14 of the datasheet. Just omit Iadj * R2 to simplify the formula, because Iadj is only 3 microamps. The two resistors setup the output voltage of the regulator. They do make some LT1963 boards that use through hole resistors.

On my module, R3 is 0R, R2 is 470 Ohms and R1 is 1500 Ohms (top to bottom). R1 on the module is equal to R2 in the LT1963 schematic. R2 + R3 = R1 in the LT1963 schematic. Using the simplified forumula (without Iadj * R2) it works out to be 5.07V. 

The above was updated based on a detailed look at my LT1963 module and the pin connections.

[OnkelPH]

[quote='hifinet' pid='52043' dateline='1707499051']
Do you mean getting the 5V from the pin 2 and/or 4 of the RPi GPIO? I don't think the voltage differential will be enough. The voltages are usually less than 5.1V, and that is right on the margin of the dropout voltage of the LT1963 to get 5.0V with the current draw of ProtoDAC. You will need at least a 100mV differential. If the voltage differential is less than indicated in the chart on page 1 of the datasheet, the LT1963 won't regulate.

Thanks, so I can get 5v from pin 2/4, but LT1963 will most likely not work. Understand.

The way it could be done is to change one of the resistors on the board. See the adjustable regulator Figure 2 on page 14 of the datasheet. Just omit Iadj * R2 to simplify, because Iadj is only 3 microamps. The two resistors setup the voltage of the regulator. They have online calculators for the LM317 which will get you very close.  Instead of having it regulate to 5.0V, you could change one of the resistors, and have it regulate at 4.7V instead, for example. The resistors can be changed by removing them or adding a resistor in parallel, to decrease the resistance slightly. On this board, they are SMD which are hard to solder. They do make some LT1963 boards that use through hole resistors.

On my module, R3 is 0R, R2 is 470 Ohms and R1 is 1500 Ohms. In the calculator above, R1 = 470 ohms and R2 =1500 ohms works out to 5.239V for an LM317. It works out to be 5.072V for the LT1963, which uses 1.21V instead of 1.25V in the formula. Note that R1 and R2 are reversed in the schematic in the datasheet. It should be as in the schematic in the calculator page.

I found a seller on AliExpress that makes them between 1.2vdc and 20vdc. Maybe an easy way to get an 4.7v

[OnkelPH]

Link to Ali [url=Just found this amazing item on AliExpress. Check it out! NOK81.53 | LT1963 LT1963A VOUT 1.21V-20V adjustable Linear, Low Noise DC Regulated Power Supply https://a.aliexpress.com/_Euh8NE5]LT1963[/url]

[hifinet]

Your link doesn't work for me.

[OnkelPH]

Try this AliExpress

[hifinet]

That link works. It looks like you need to supply the SMD resistors and solder them. "It will send 5V by default." You might as well get the other one. It depends on how well you can solder SMD. It looks like the resistors on my module were soldered by hand. If I can find the module that uses the through hole resistors, I will post it.

[hifinet]

They look like through hole pads, but they are just SMD pads with through hole parts soldered in place. Very tricky. There are a few different versions of these boards.

[OnkelPH]

That link works. It looks like you need to supply the SMD resistors and solder them. "It will send 5V by default." You might as well get the other one. It depends on how well you can solder SMD. It looks like the resistors on my module were soldered by hand. If I can find the module that uses the through hole resistors, I will post it.

Thanks.

[hifinet]

The difficulty in adjusting the voltage on these is that you are working with standard value resistors. It's sometimes easier to add resistors in parallel to the 1500 ohm to get a value that is close. For example, with a 13,500 ohm resistor in parallel with the 1500, the resistance is 1350 ohms, and the voltage output becomes 4.69V with the 470. 

In this case, I would add a resistor to substitute for the 0 ohm R3 (top resistor). Resistors come in standard values. A 47ohm in place of the R3 (470 + 47) would get you 4.72V with my module, give or take. Check my math.

I have some other LT1963 modules purchased earlier. Much nicer reflow soldering of the resistors. Also, the values of all three were visible. R3 471 (470 ohm), R2 0, R1 152 (1500 Ohm). Got some measurements. They are 0603 (in) or 1608 (mm) in size. That's 1.6mm length and 0.8mm width.