TPS62170 low efficiency at low currents

Hi Chris - Not sure how to post but I will send the schematics and a spreadsheet of the efficiency if you let me know how.

Thanks Chris

Here is the data

Input voltage VIN= 5V
Test items			load current (mA)	VIN (C56.1)	VIN Current(mA) (J3.1)	PP4V4 OUT Voltage (C11.1)	PP4V4 OUT Current(mA) (R12.2)	Efficiency (%)
High Voltage Buck PP4V4		250	249	5.00169	229.5198	4.37695	249.1885	95.008542
		200	199.3	5.02293	182.8042	4.37714	199.3609	95.03577652
		150	149.4	5.00401	137.8649	4.37715	149.5417	94.88157037
		100	99.5	5.00778	92.62563	4.37341	99.64954	93.95482234
		50	49.6	5.01865	48.34521	4.37296	49.77986	89.71990717
		40	39.6	5.00082	39.55464	4.37336	39.7952	87.98472098
		30	29.6	5.00525	30.12694	4.37548	29.83321	86.56550988
		20	19.7	5.00333	20.58255	4.37675	19.91097	84.62249603
		10	9.7	5.0093	10.89389	4.3798	9.922079	79.63370691
		8	7.5	5.00649	8.902365	4.38161	7.919515	77.85626317
		6	5.8	5.00464	6.994271	4.38254	5.951071	74.50848316
		4	3.6	5.00745	4.937832	4.38549	3.946546	69.9974732
		2	1.8	5.00699	2.950119	4.38626	1.944892	57.75287419
		1	0.7	5.00356	1.94571	4.38691	0.9569108	43.11942471

Your schematic didn't come through. Can you try it again?

Do you need a 5Vin to 4.4Vout converter? Is there a particular reason that you use a 17V rated converter?

Hi Chris

We have two supplies: Normally from a 12v source, the second is from a USB supply hence the TPS62170.

I tried to send the schematic in PDF format. The paste from word button didn't work? Should I use aanother method?

Did the schematic accompany this submittal?

Pete Peterson said:

I tried to send the schematic in PDF format. The paste from word button didn't work? Should I use aanother method?

Pete,

There are two different options currently available for attaching files. See this article here for a how-to and walk through of each option.

Thanks Blake for helping!

Thanks Pete for posting. Let me check it and reply back.

I built up your circuit on an EVM and checked the 3 lowest output current points against your readings. I measured lower Iin values, which would give better efficiency than what you recorded. So, let me make some comments/observations:

There is no 4.4Vout data in the D/S, so it is difficult to compare against the existing data for what the efficiency should be. As well, your inductor has a DCR about 3x that of the inductor used on the EVM, which would give a few points lower efficiency anyways.

During my testing, I noticed that with a little bit below 5Vin, the IC's current consumption and behavior was different. So, can I confirm that you measured Vin and Vout across the capacitors near the IC?

With a little bit lower Vin, the IC was in PWM mode instead of power save mode as it should be at the lower currents. You can check if this is the case where your measured efficiency is low. This is because Vin is so close to Vout and the duty cycle is so high. Thus, the needed off time is very short. In this short off time, the reverse voltage discharging the inductor is too long and so the inductor current goes negative. This steals energy from the output caps which means the output voltage drops a little. Then, the IC has to immediately switch again to get it back to where it should be. This is PWM mode with some negative inductor current which is less efficient. A larger inductance should boost efficiency by staying out of PWM mode by decreasing the slew rate of the inductor current. You could try 3.3uH or 4.7uH and see if your results are better. The higher DCR compounds this issue by requiring the on time to be a bit longer to overcome that IR drop.

In any case, I'm not sure how critical the efficiency is at light loads when operating off of a USB port. It is typically a bigger care about at full load to keep the thermal rise low. It's also important for battery applications to make the battery last longer, but it doesn't sound like you have a battery in your application.

Finally, it's important to consider what is the efficiency of the TPS62170 circuit vs. the rest of your system. For example, there is a resistor divider hanging from Vin to EN to GND. This draws some input current. Other circuits on your PCB that are also powered from the 5V will draw current as well. This lowers the efficiency.

Lastly, the FB divider resistors you use are lower than needed for the TPS62170. This lowers efficiency at very light loads as explained here: www.ti.com/.../slyt469.pdf

Hi Chris

Well you certainly have addressed the issues for me for which I am grateful. My circuit is not battery powered and I have been leery of the 2.2uH inductor. You are right , the efficiency at higher currents is certainly more important for temp. rise issues etc.

I will try the 3.3uH inductor and see if it props up the efficiency a bit.

Thanks for all your help - TI is lucky to have you!