Importance of 0V regulation?

[Mephitus]

I have been working a very small footprint bench supply based on the LT3081 and LT3092 (0-25V 3A). Current revision spice simulation attached. The LT3081 can regulate down to 0V output with a 8ma minimum load. I have foud some ways of doing this with a LTC3632, pass transistor, and mosfet. But the extra component cost brings the BOM cost up $6-7 and would require a couple more square CM's of PCB space.

How important is proper 0V regulation anyhow?
Especially considering the very low 8ma current draw required to achieve this.

[mikerj]

Can the the output be driven to zero when the supply is current limiting?  If not then it may not provide protection in the event of a dead short.

[Mephitus]

Can the the output be driven to zero when the supply is current limiting?  If not then it may not provide protection in the event of a dead short.

The LT3081 has built-in current limiting and can take a dead short without a problem indefinitely. Here is the datasheet. It's a pretty nice little IC.
http://cds.linear.com/docs/en/datasheet/3081fc.pdf

If the current limiting pin(s) are grounded, it automatically self-regulates the current. I have 2 configured in parallel and sharing the current load (1.5A per chip, 3A total). I have a pot set up to control it manually too. I am cleaning up the schematic and will post it once it looks presentable.

[Zero999]

What are you trying to do? Eliminate the minimum load requirement of the LT3081?

What about using a current mirror?

[Mephitus]

What are you trying to do? Eliminate the minimum load requirement of the LT3081?

What about using a current mirror?

I am just wondering if it is worth the effort, pcb space, and increased cost to put an -8ma load on the output to meet the minimum current output required for 0V operation?
(Current revision attached)
EDIT: Typo on R3, should be 3.92K. R4 should be 100

[Zero999]

It needn't be complicated. R1 just needs to pass more than 4mA. Tr2 will also pass approximately 4mA down to fairly close to 0V.

[Mephitus]

It needn't be complicated. R1 just needs to pass more than 4mA. Tr2 will also pass approximately 4mA down to fairly close to 0V.

I was unfamiliar with that. Thanks! I will add that in next time I have the chance. 

[Zero999]

I was unfamiliar with that. Thanks! I will add that in next time I have the chance. 

It's a current mirror. A very common circuit inside analogue ICs such as op-amps and voltage regulators. There are probably many current mirrors inside the LT3081. They could have easily added another current mirror to the LT3081 to remove the requirement for a minimum load, but it would increase the current consumption, so would be a bad idea in applications which will always draw over 4mA.

[SeanB]

A useful tip with a current mirror is to use 3 identical transistors in the unit, and put 2 in parallel, so that the output current is roughly 2 times the input current ( saving power) or use the output collectors as separate current sinks. Works quite well, providing the 3 transistors are all from the same lot and are in good thermal contact with each other. Inside an IC this is determined by the ratio of the active areas of the transistors ( roughly) so you can made a whole load of current sources using one diode and high value resistor ( mostly in IC use a pinched off Jfet instead of a resistor which is hard and large to make in silicon plus it is very poorly defined and has a horrid tempco) and a multiple of sharing transistors, often of varying area to get current ratios that are correct for the design.

[Zero999]

A useful tip with a current mirror is to use 3 identical transistors in the unit, and put 2 in parallel, so that the output current is roughly 2 times the input current ( saving power) or use the output collectors as separate current sinks. Works quite well, providing the 3 transistors are all from the same lot and are in good thermal contact with each other. Inside an IC this is determined by the ratio of the active areas of the transistors ( roughly) so you can made a whole load of current sources using one diode and high value resistor ( mostly in IC use a pinched off Jfet instead of a resistor which is hard and large to make in silicon plus it is very poorly defined and has a horrid tempco) and a multiple of sharing transistors, often of varying area to get current ratios that are correct for the design.

That's a good idea. Transistor arrays or matched pairs can be purchased for making current mirrors.

A quad NPN array could be used with Tr2 being made of three transistors connected in parallel to multiply the current by three.