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LMR14010A: +/-16.5V Inverting Buck Boost Application

Part Number: LMR14010A
Other Parts Discussed in Thread: LMR36015

Hello TI,

I have designed an IBB with LMR14010A. I used this application note as a reference.

Design is as follows:

-16.5V IBB Design
VIN 16.5 V
VOUT -16.5 V
IOUT 0.5 A
MAX V 33 V
FREQ 700000 Hz
ΔIL Chosen 0.15 A
EFF. 0.82  
L Calc. 0.000086 H
L Chosen 0.0001 H
ΔIL Actual 0.130 A
IPK 1.175 A
IAVG 1.110 A
Regulator LMR14010A  
Current Lim 1.5 A

I have built the design, and now I'm validating, but I'm running into an issue.

The design is meant to handle 0.6A at -16.5V, but the actual use case will be approximately 1/2 of that (0.3A). Of the 3 samples that I built, all begin to behave abnormally before 0.6A, and 2 of them before 0.5A.

Here is my best sample at 500mA out.

As you can see, it is acting properly. The ripple currents as almost spot on my calculation. As I increase the current, it eventually starts looking like this:

At first, I thought maybe I was saturating, but looking at the the peak current, it doesn't seem to be the case. The inductor is rated for 1.7A (continuous and saturation). I also initially thought, maybe I'm hitting the 1.5A current limit of the part, but again, this seems to be within that. Another thought was possible the IC was hitting the temperature limit (170 at the junction). Heat does exacerbate this issue. I have seen the case getting close to 100C. Heat may be the main culprit, and how much this part is trying to dissipate. At 0.5A, I'm seeing ~80% efficiency which correlates to ~2W being dissipated. Again, this may be the main culprit here, but I want to make sure this isn't a subharmonic oscillation?

When in this mode, increasing the input voltage resolves this issue, making me wonder about this possibility. So this is my main question. Does this look like a sub harmonic oscillation to you, have you run into this, and is there anything that can be done or not since this part has built in slew rate compensation? Being a simple switcher is nice, since it's simple, but maybe not so nice since I cannot do anything about the compensation network.


  • Hello Joshua,

    It appears from your waveform that you are approaching peak current limit of the device being 1.5A typical. I cannot make out the exact value but it appears to be approaching 1.5A

    I can see if I can locate the spread on the current limit.

    You may want to increase fsw if your application allows to reduce ripple and/or increase inductance of output filter.

  • Marshall,

    Thanks for the feedback. As you can see in the second plot, I'm reaching 1.35A PK. If the tolerance of the current limit is 10%, than yes, this could be a possibility.

    However, on a "worse" sample that I tested, the max was only 1.235A PK. So tolerance would need to be even worse than 10%. See screenshot below:

    Does this look like subharmonic oscillation to you? As a side note, I have tried doubling output inductance, but that made no change. Again, increasing input voltage rectifies the issue.

    [EDIT] Yes, I am looking into other regulator options. As you are aware, this regulator is a fixed frequency regulator (700kHz), and I'm trying to understand what is happening before I move on to another part, and understand if there is anything than can be done to resolve this issue since the compensation is done internally.


  • Hello Joshua,

    You bring up a good argument based on your observation. It does appear you are approaching 50% DC and behavior can be characteristic of SHO, though your inductor looks quite reasonable for the application.

    Based on the facts you mentioned, perhaps it may be a stability issue.

    For these types of designs, it can be a little tricky sometimes with the RHP zero that comes in to the picture for IBB.

    What is your output cap? Have you tried manipulation of it.

    Unfortunately, my modeling tools are quite limited for this device. If you are flexible in device  selection, I may be able to better support say if you were to select LMR36015 for IBB config.

    We can discuss over email if your interested. I can get an EVM in your hands.

  • Also CFF may be needed if you are not already using one.

    22pF is good start. I had to use CFF approaching 1nF one for an IBB design! 

    CFF will help mitigate the phase loss associated with the RHP zero.

    Right now I am having difficulty finding derivation of RHP zero...I want to say it is the same as buck boost, but I am not positive.

  • Hi Marshall,

    Thanks for this feedback. My output cap is a 330u 25V UCV series Panasonic cap. From my calculations, this far exceeds what is required to meet voltage/current ripple requirements, as well as transient load response. It was chosen because it is a part I have on hand, otherwise, I would have likely picked something much smaller. I have a 1u ceramic point blank followed by this capacitor. Per your suggestion, I tacked on a 1500u, 35V cap, but observed no discernable change to the switching waveforms.

    When I had the demo's boards built, I had a 470p in the CFF position. This proved to be problematic, causing erratic behavior at light loads, that looked very similar to the scope shots previously posted. I currently have no cap in this position, but I did try to place a 10pF early on to see if it would help, but again, no change. I have not tried other values for this cap however.

    I'm open to changing to another regulator IC, but I want to fully understand why the present IC is unhappy. Also, and I know this is very unpopular, but I would prefer an IC with leads. I have been bit by leadless parts. They are certainly the future, I'm just resisting as long as possible.

    Best Regards,


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