Dave’s uSupply Battery Powered Bench PSU rev-C

[gregfox151@gmail.com]

Hi all,
I bread boarded a portion of Dave’s uSupply Battery Powered Bench PSU rev-C and I think I found a problem. (See attached) If the LT3080 does not have a current draw on its output (pin 3)  of a least .5mA it will not stay in regulation. Dave added a LM334 to the output to keep it in regulation, but the problem is the LM334 operates only down to 1 volt, therefore as the voltage drops down below a volt the LM334 will no longer operate, and the LT3080 will be out of regulation (you can’t get it below ~2 volts)
In the attached schematic I show the bread board, and I can only get it down to 2.4 volts even after grounding the set pin.  Does anyone know how to keep it in regulation and get “0” volts as an output? Does this work for Dave?
Will he ever produce this product?
Thanks!

[kizzap]

If I recall correctly, it says somewhere on the datasheet that the current needs to be sunk to a negative rail for the thing to work correctly.

-kizzap

[minime72706]

I vaguely remember the video blog he did regarding using a current source to ensure minimum load for regulation - did he address the issue? I think he did.

I can't think of an ELEGANT solution off of the top of my head.

[kizzap]

From the LT3080 datasheet itself:

Output Voltage
The LT3080-1 generates a 10µA reference current that flows out of the SET pin. Connecting a resistor from SET to ground generates a voltage that becomes the reference point for the error amplifier (see Figure 1). The reference voltage is a straight multiplication of the SET pin current and the value of the resistor. Any voltage can be generated and there is no minimum output voltage for the regulator. A minimum load current of 1mA is required to maintain regulation regardless of output voltage. For true zero voltage output operation, this 1mA load current must be returned to a negative supply voltage.

[gregfox151@gmail.com]

Thank all of you for the reply.  As you can see from the schematic, SET was grounded (0 volts). I decided to try other 3080s and on the third try it worked, bad batch.

[minime72706]

From the LT3080 datasheet itself:

Output Voltage
The LT3080-1 generates a 10µA reference current that flows out of the SET pin. Connecting a resistor from SET to ground generates a voltage that becomes the reference point for the error amplifier (see Figure 1). The reference voltage is a straight multiplication of the SET pin current and the value of the resistor. Any voltage can be generated and there is no minimum output voltage for the regulator. A minimum load current of 1mA is required to maintain regulation regardless of output voltage. For true zero voltage output operation, this 1mA load current must be returned to a negative supply voltage.

Despite this, you can typically get VERY CLOSE to 0V without returning it to a negative rail.