How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

【Introduction】With the development of LED lamp bead technology, compared with traditional analog dimming technology, digital dimming technology has made great progress in recent years. There are thousands of dimming products to choose from in the lighting market today, and we need to take these factors into consideration when choosing a dimming driver. Dimming smoothness, dimming depth, whether there is perceptible stroboscopic and ripple during dimming.

With the development of LED lamp bead technology, compared with traditional analog dimming technology, digital dimming technology has made great progress in recent years. There are thousands of dimming products to choose from in the lighting market today, and we need to take these factors into consideration when choosing a dimming driver. Dimming smoothness, dimming depth, whether there is perceptible stroboscopic and ripple during dimming.

In order to achieve the ultra-fine smoothness of the dimming output, it is first necessary to understand the difference between each dimming level. The smaller the difference between each dimming level, the smoother the dimming. This enables stepless dimming during the entire dimming process. As shown below.

How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

The case of this article is a customer who needs to quickly measure the linearity and smoothness of the output current during PWM dimming, and then enter the integrating sphere to test the brightness linearity. The device under test is a highly integrated, dual-mode buck with constant voltage or constant current output and carrier two-bus chip with non-polar access. At the same time, customers are worried about whether this kind of dimming chip with bus function will affect the LED dimming during bus communication. The switching frequency of the Buck part is as high as 1Mhz, which brings a lot of challenges to the current measurement. This paper chooses to use the TCP0030A probe, this probe can reach 120Mhz bandwidth, which is very suitable for the measurement of high-speed current signals.

We will take these questions and enter the test and verification together.

Testing purposes:

1. Use the signal generated by AFG31252 for PWM dimming, and use DMM6500 to scan the output current linearity.

2. Use a current probe to measure the Inductor current to see if the inductor is saturated during dimming, so as to select a suitable inductor.

3. Observe whether the dimming is affected during the bus communication, and whether it causes the LED to flicker.

The test environment has been set up:

How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

Test ideas

How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

Let’s test problem 1 first:

Use AFG to generate PWM to adjust the duty cycle. Here, instead of connecting a multimeter into the loop, use the DM6500 to measure the voltage of the current-sense resistor and calculate the average value of the string current. This results in the following chart:

How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

From this point of view, the linearity is still excellent in the full duty cycle dimming range. In each PWM cycle driven by the AFG, the SW can respond quickly, and the current reaches the preset value without overcharging.

How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

We then verify whether the LED current will be affected during bus communication. Thanks to the TCP0030A’s 120Mhz speed, we can observe possible, very small current noise. This provides very clear measurement support for finding some inexplicable LED flickering, micro-brightness and other anomalies.

In fact, it can be seen from the above figure that during the off period of the dimming PWM signal, the carrier waveform on the bus does not generate any spikes on the LED current. Let’s take another look at the situation during the EN on cycle.

How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

It can be seen that when the bus communication waveform changes rapidly, the light string current still does not have any burr or overshoot.

Let’s change the current probe to the inductor and observe the inductor current.

How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

It can be seen that the current can reach up to about 900mA during the open cycle. Thanks to the storage depth of TBS2000 as high as 20M, the waveform is still very clear after ZOOM. It can be seen that the currently selected inductor does not have current saturation phenomenon. Of course, in order to more carefully evaluate the availability of the inductor, it is necessary to do more testing of the inductor current in various switching modes, see the previous article. The rise of this current is only 640ns! This is a constant current LED driver chip with a switching speed of 1Mhz. Low-speed current probes cannot be measured. For the measurement of such signals, this TCP0030A current probe with a speed of up to 120Mhz is almost the only choice.

The customer then uses the serial port to transmit the MODBUS protocol on the second bus, and uses the MCU to control the brightness of the light string according to the frequency given by the AFG, and soon completes the design of a lighting solution.

How to test LED driver dimming linearity using oscilloscope, AFG and multimeter?

(Source: Tektronix)


The Links:   LQ150X1LW72 2MBI400TA-060

Related Posts