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LED Drivers/LCD Bias
LED Drivers/LCD Bias Forum
The efficiency of the demo board isn't what I would expect. At 6V in it draws 1.5A and the LED current is only 0.5A. Everything seems to be working, output voltages, over-voltage etc. The waveform at TP1 is a square wave switching between 0 and about 17V. 800nsec at 17V, 1.4usec at 0V. I'm thinking the input current should be closer to 0.6A. What can be wrong?
You will need to measure efficiency without PWM dimming. You can then do a weighted average to calculate your operating efficiency. Without PWM dimming, you can expect to get the measured results published in the TPS61500 EVMs User's Guide. Refer to http://www.ti.com/lit/an/slva236/slva236.pdf for more information on efficiency measurements. If you are unable to replicate the EVM efficiency measurement, we will need your exact test conditions and a schematic of your exact test setup.
Michael: This is an eval board and the PWM dutycycle is 100 % which is selected with a jumper on the board. The currents were measured according to the recommended mode with a large capacitor on the input (Buck mode) and a current meter. The currents were also confirmed with a scope current probe to confirm that the meter readings were not bogus. Wondering if there can be something wrong with the chip, although all the features seem to function although I haven't tried any analog pwm yet.
You verified your response, so I'm assuming you figured it out. What was the problem?
Actually I didn't, I wasn't sure what the verify did... Now I know!!
No problem. We ask the original poster to verify a post when it clarifies their question or solves their problem. Can I ask you attach a file with your measured data so we can take a closer look at your measurements. Also, what efficiency were you expecting?
I didn't actually record the earlier readings as they were so far off. I just remeasured them again with the input voltage at 4.998V. I have a 46uF cap on the input. With a B&K 390 VOM I measure 2.10 A input. With JP2 open I measure 0.50A LED current. With JP2 installed I measure 199.0 mV across R8 which is current sense resistor. Everything seems to be working but the current is to high. I would have expected efficiencies in the 80% range implying the input current should be about 0.5-0.6A at least. Not 2A I will look a
Efficiency measurements require Vin, Iin, Vout, Iout measurements. Your Vout can vary significantly depending on the LED temperature, V-I characteristics, etc. Let me know your specific measurements and we'll be able to start working from there. Note that you need to use a current meter to measure Iout. If you use the current sense resistor to calculate Iout, you'll first need to calibrate your calculations to match the resistor's exact resistance value. That can vary +/- 1% due to initial tolerance from the manufacturer, plus additional amounts due to soldering, moisture, aging, heat, vibration, etc.
Michael: It finally dawned on me that we are talking about Power. I was focused on current which is obviously incorrect... DA. Thanks for you help and enlightenment.
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