I have a LM22679-ADJ based buck converter. The output voltage is set to 24V. The input voltage may vary from 28-40V DC. The current limit is set to above 5A according to the graph given in the datasheet. Here's the schematic:
*The values of R1,R2,R3,R4 were incorrect. Actual values used were 47k,47k,5.1k,1.8k. The R7 resistor is set to 6.8k to set load limit at just above 5A. And one of the inductors is shorted. Diodes used were Schottky type as advised in the datasheet.
- With a load of 25 ohms the waveform across D1 becomes distorted, and the device starts limiting current, when measured across the diode D1 only when the input voltage is above 31V.
- When I add another inductor in parallel to the one in the circuit thereby reducing the inductance to ~2.3uH, the input voltage at which the output voltage drops out of regulation increases, however, it is not completely gone.
The following are the images of the DSO under different circumstances, all measured across the diode D1 and load resistance fixed at 25 ohms.
Case 2: Inductance 4.7uH. But input voltage increased to 35V. Although output voltage is very close to set voltage, the waveform doesn't make sense. What is supposed to happen is the square wave is supposed to reduce its duty cycle. But this happens instead.
Case 4: Inductance 4.7uH. Input voltage increased further to 37.5V. The output voltage completely drops out of regulation to about 17V. To reset the IC from this state, either the load must be removed or it must be reset at lower input voltage. Simply reducing the input voltage to where it previously used to regulate output does not make the IC regulate output again. There is a hysteresis of some sort.
I have calculated the output inductor value as per the datasheet and it came out to be 9.6uH for max input of 40V and output of 24V. But, with both inductors in series, the output dropped out of regulation much sooner than in the above mentioned cases. This is the datasheet.
- Why is the wave form like this and not a square wave like the first case at higher input voltages? (and what is the role of the output inductor in this)
- What can I do do that my regulator does not drop out of regulation for the max input of 40V?
EDIT: @csabahu I had the chance to take some measurements today.
At 10uH,200uF,25Ω the IC drops out of regulation at input of 32.2V
At 4.7uH,200uF,25Ω the IC drops out of regulation at input of 36.2V
At 2.3uH,200uF,25Ω the IC drops out of regulation didn't drop out of regulation for 40V. But I tried a load of 16.7Ω and it certainly did drop out of regulation.
Halving output capacitor value had no significant effect on above measurements.
I also tried increasing the inductance value to about 24uH. At 100uF and 25Ω there was no dropping out of regulation. Further, the ringing due to what I'd presume was discontinuous mode operation was completely gone even at 40V.(Which is to be expected I suppose) This also holds for 16uH at similar conditions. However, at increasing the load to 16.7Ω the IC immediately goes into current limit. What was interesting to note here was that the IC seemed like it was skipping cycles. And the waveform did not resemble the DCM operation. The frequency was lower than the standard 500Khz.
I removed the 6.8k R7 current limit resistor, but this had no effect on any of the above measurements.