Low cost buck SMPS with IC AOZ1284 - 5V x 4A - values for COMP pin

Question

I want to use a buck converter based on IC AOZ1284PI from Alpha & Omega Semiconductor. AOZ1284PI is a low cost buck controller. For example, on Digikey it costs USD 0.51 @ 500 whereas the TPS54340 from Texas Instruments costs USD 2.53 @ 500. The IC support up to 4A of continuous output current, so I want to use it to reduce a 12V rail to a 5V rail capable of delivering 4A. Below are the parameters that I used to make the calculations using the equations shown in the datasheet. I have calculated RC to be 180K ohms e CC to be 1.5nF. I would like to ask if somebody can do a revision on these calculations, and check if these values are really suitable, the value of CC and RC on schematic, that goes connected to COMP pin.

Datasheet: http://www.aosmd.com/res/data_sheets/AOZ1284PI.pdf

Parameters:

• Vfb = 0.8V
• Vout = 0.8 + (0.8 * 1800/330) = 5.15V
• Iout = 4A
• CO = 141uF
• COesr = 5 mOhm
• inductor = 4.7uH 8x8x4mm (SRN8040TA-4R7M). Current 5.8A. Saturation 6.7A
• FSW = around 1MHz (47K 1% resistor)
• fc = 30KHz (crossover frequency) or maybe 5khz if better
• Vin = 12V
• RL = 5.15V / 4A = 1.28 Ohm (datasheets states RL)

Calculations:

• fp1 = 1 / (2pi * CO * RL)
• fp1 = 1 / (6.28 * 141 uF * 1.28 Ohm)
• fp1 = 1 / (6.28 * (141*10^-6) * 1.28)

• fp1 = 882.2

• fz1 = 1 / (2pi * CO * esrCO)

• fz1 = 1 / (2pi * 141 uF * 5 mOhm)
• fz1 = 1 / (6.28 * (141*10^-6) * (5*10^-3))

• fz1 = 225866.2

• RC = fc * (VO/VFB) * ( (2pi * CO) / (GEA*GCS) )

• RC = 30000 * (5.15/0.8) * ( (6.28*(141*10^-6)) / ((200*10^-6)*(4.5)) )
• RC = 190009.25

• RC = 180K...

• CC = 1.5 / (2pi * RC * fp1)

• CC = 1.5 / (6.28 * 180000 * 882.2)
• CC = 1.5*10^-9
• CC = 1.5nF...

Then, finally

• fp2 = GEA / (2pi * CC * GVEA)
• fp2 = (200*10^-6) / (6.28 * (1.5*10^-9) * 500)

• fp2 = 42.4

• fz2 = 1 / (2pi * CC * RC)

• fz2 = 1 / (6.28 * (1.5*10^-9) * (180000))
• fz2 = 589

Schamatic:

Below I show my current layout, from top layer:

On the bottom layer, I have a good ground poor.

Regards.

EDIT1 Schematic and layout after user Verbal Kint answered to the question. 12V input, 5V output @ 4A

Answer 1

As I said in the comments, before attempting to stabilize a converter of any type, you need its control-to-output transfer function. The problem here is that the data-sheet of this Alpha & Omega chip is eloquently empty so difficult to figure out what its internals are. Anyway, I have captured a schematic using Elements, the SIMPLIS free demo version in which I included the output caps. ESRs and the inductor resistance:

The chip can switch up to 6 A with a 0.22-\$\Omega\$ internal sense resistance and there is probably some internal slope compensation but there are no details. To compensate this guy, I have automated calculations as shown in the book I wrote a while back:

From the simulation, you first verify the operating point is ok, meaning the converter regulates and delivers 5 V from the 12-V input source:

The circuit switches at 1 MHz and delivers 4 A to the load. The feedback voltage is around 900 mV and we can extract the control-to-output transfer function now:

If we pick a 10-kHz crossover frequency, we extract the following data from the graph: the magnitude at \$f_c\$ is -5.8 dB while the phase is -82°. Enter these data in the automated sheet and run the simulation again. Look at the compensated look gain and check it is ok:

Oui ! Perfect, a 10-kHz 0-dB crossover frequency with the wanted 70° phase margin. A rock-stable design for this operating point. You must now explore various situations (load changes, input voltage changes, output cap. value and ESR spread etc.) to make sure the stability is not at a stake in any of these situations but for a simple project it should do well. The calculated components values for the COMP pin are: \$R_2=76.4\;k\Omega\$, \$C_2=55\;pF\$ and \$C_1=835\;pF\$. You can round these values to the closest normalized values of course. Good luck with this design!