LP2985 is Not Working

Igor Martinovski50

Other Parts Discussed in Thread: LP2985

Hi,

I am attempting to use the LP2985 for a 5V analog supply. However, when I apply the 12V input voltage, the output sits at 1.25V and is stable. (internal reference voltage perhaps?). The load should be less than 100mA. however I have not tried disconnecting the load yet.

The capacitors are X7R ceramic. I realize their ESR is probably too low.

When I removed C316 and any immediate downstream capacitors, the problem persisted however this time the voltage ramped to 1.25V slower.

I also replaced C316 with a 10uF tantalum cap and the behavior is the same.

I replaced the LP2985 with a new device and the behavior was the same. Layout is not exactly per the datasheet but the parts are reasonably close.

Any suggestions?

over 8 years ago

0 Michael J Schultis over 8 years ago

TI__Mastermind 24485 points

Hello Igor,

As you mentioned there are a couple of things that you need to be sure of:

The capacitors and what their ESR ratings should be is pretty well documented in the datasheet in section 8.1.3. Additionally Figures 18 through 20 give stability regions based on capacitance and ESR. We would typically suggest that a ceramic be used, but if you are concerned that the ESR is too low, then make sure that the ESR for your capacitor is not below the stable region in these figures.

Also, as you mentioned it is important that the capacitors be close to the device. Specifically, the output capacitor should be within 1cm of the device.

Another point to consider is if the part is getting too hot. You mentioned that there is a 7V drop and that the current may be up to around 100mA or so. The thermal resistance of the device is 206C/W, so please take this into consideration.

Best,
Michael

0 Igor Martinovski50 over 8 years ago in reply to Michael J Schultis

Prodigy 165 points

Hi Michael,

Thanks for your reply. The part does not get hot or warm. The capacitor is fairly close to the device (see layout below). I think it is an issue with the ESR of the output capacitor. I will try a different capacitor.