MIC29312 based linear driver

[Jufran88]

So, after designing a LMS1587 based linear driver rhd introduced me to the MIC29312 IC. I already sent my LMS1587 design to the board house and I couldn't get this in because it passed the deadline for submission. There is plenty of time before the next deadline so I could still tweak the design if needed.

data sheet: *here*

It's an ultra low drop out linear regulator with better specs than the LMS1587 and includes an enable switch that can be used as a safety switch. If not then it can be soldered with V+.

Specs:
Regulator Topology: Positive Adjustable
Voltage Output: 1.24 V ~ 15 V
Voltage Input: 2.3 V ~ 16 V
Voltage Dropout (Typical): 0.6V @ 3A
Number of Regulators: 1
Current Output: 3A
Current Limit (Min): 3A
Operating Temperature: 0°C ~ 125°C
Mounting Type: Surface Mount
Package/Case: TO-263-6, D²Pak (5 leads + Tab), TO-263BA

Schematic:

Board:

* It might be hard to see all the connections because some parts are connected to the ground plane.

comments, questions, concerns are welcomed!

 

[Wolfman29]

Doesn't that say 3A min current? If so, we can't use that to power any of our diodes.

 

[Jufran88]

That refers to the current limit. So at minimum it's limited to 3A, but it has allowance for more current passed 3A.

 

[rhd]

I'm excited to see this come to fruition. When is the next deadline?

(and for the record, I'm yet to test this IC myself, so if it fails horribly, my apologies!)

 

[Wolfman29]

Oh, alright. Makes sense. Thanks for the clarification!

 

[Jufran88]

@rhd:
I know the wait is killing me! :banghead: The deadline is April 4 which is a week away plus another 9 days . It's worth the wait because the boards are very high quality! I could send you a sample when I get the boards just pay me for the shipping costs.

@Wolfman29:
No problem! How is your LM317 design coming along?

 

[rhd]

Jufran - is there any price-point break for ordering more?

Because I would gladly pay for a few of the prototype boards. I lack the skillset to properly design PCBs

 

[Wolfman29]

I've had a nice, easily makeable, Aixiz module fittable LM*1117* design for quite a while now (I just cut a through-hole board and just solder on the pieces, I can make it fit an aixiz module ). I am scanning through possible other regulators though to see if I can improve upon the common design.

 

[Jufran88]

@rhd:
Sure, I could order some for you along with mines. The boards come in sets of 3 and they charge $5/ sq in. These boards are pretty small so they only cost a few dollars. :beer:

@Wolfman29:
Awesome, there are a lot of possible linear drivers! I found myself stuck for a whole day looking for some.

 

[rhd]

Really? That would be phenominal - I'd be so proud to own a bunch.

Am I right to assume that they would be less than a square inch each... so that basically means $5 for 3? So like $1.66 each? Wow...

I'd grab like 10 of them Errrr.... I guess 9?

 

[Jufran88]

There's another place to get prototyped PCBs that charge $2.50/ sq in. but they have a $10 setting up fee and a fee for shipping. It would be more convenient for bigger orders, but for small orders Laen's is pretty good. There's a link in my sig if you want to check it out.

 

[Jufran88]

Ok, so my boards came in friday afternoon and I just got to testing it earlier today.

I'm having problems with the current. I have (2) 4.3 ohm resistors in parallel to give a total of 2.15 ohms (measured at 2.3 ohms) together.

Using the Vref/resistor equation I should be outputting:
1.25V / 2.3 Oms = .543A or 543mA

However, on my test load I only get 9mA. I've tried using different input voltages and different settings on my test load, but I seem to not be able to reach more than 100mA.

Enable and V+ are connected to the battery positive as suggested by the datasheet.

Does anyone have any suggestions? :can:

 

[jib77]

What is you Vin? What is you dummy load Vfw?

Also nominal Vref for this chip is 1.24V, but that doesn't explain the problem here nor does it make a significant difference in the calculation(539mA).

Its either starving for voltage or going into thermal shutdown ... you may not have enough thermal pad mass. Unfortunately the datasheet doen't have much thermal design info :/

BTW I too use dorkbot PCB :beer:

 

[Jufran88]

Laen does an awesome PCB service! :beer:

My Vin was 6.08V and I also tried it at 7.4V. I set my testload at a 2.8V (.7*4=2.8V) and 4.2V (.7*6=4.2).

I don't think its going into thermal shutdown since it's not even warm. Maybe while I was soldering I may have over heated the IC and fried it?

I tried testing the voltage bewteen the LD+ and LD- and it puts out 6.08V, same as my input voltage.

I actually made the pad bigger compared to the design specs on the LMS1587 IC. However, it is a little bit bigger than the LMS1587 even though they are the same package by an extra 2mm.

 

[rhd]

Note 10: VEN ≤ 0.8V and VIN ≤ 8V, VOUT = 0.

With 2x lithium ion cells fully charged, you might be overloading the VEN. However, you mentioned trying 6V and 7.4V, so that's probably not it. (However, once we've solved our problem here, I would probably suggest adding a diode between VIN and VEN on future versions, to drop the 8.2V 2x cell input voltage on the VEN pin down to below 8.

 

[rhd]

LM317 Datasheet Formula:
Vout = 1.25 (1+ R2/R1) + Iadj(R2)

MIC Datasheet Formula:
Vout = 1.24 (1+ R2/R1)

Notable difference being the lack in the latter of an Iadj(R2) figure. Is this relevant?