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Vout = 0.8 * (1 + R1/R2)
5V output requires standard resistor values of: R1 = 52.3k, R2 = 10k (recommended from datasheet) this yields around 4.984V
alternatively, R1 = 53.6k yields around 5.088V (Would it be better to design for slightly higher than 5V for some error compensation?)
Even better, 10.5K, 2K gives exactly 5V (strange that this was not recommended by the D.S)“The combination of R1 and R2 should be large enough to avoid drawing excessive current from the output, which will cause power loss” - This says nothing about the resistor values being too large so I will go ahead with larger values as it allows for a more accurate 5V rail
given this case: 105k and 20k would also be ideal
standard resistor values source: https://www.rfcafe.com/references/electrical/resistor-values.htm
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Vin = 20 - 26V
Vout = 5V
Fs ~ 1MHz
Ripple Current estimation: IL = (0.2 to 0.4) * 4A = 0.8 to 1.6
Inductor should be rated to minimum 5A to handle the peak current in the above eqn
L should be between 2.34uH and 5uH
After EE meeting, estimation of 0.3Iout for ripple current, inductor should be around 3.125uH to around 3.3uH (same calculations as above)
standard inductor values around this range: 2.4, 2.7, 3.0, 3.3, 3.6, 3.9, 4.3, 4.7, 5.1
https://www.rfcafe.com/references/electrical/inductor-values.htm
will choose a 4.7uH to satisfy the upper bound inductance requirement (original decision)
new choice would be 3.3uH
https://www.digikey.ca/en/products/detail/sumida-america-inc/0630CDMCCDS-4R7MC/5812458
https://www.digikey.ca/en/products/detail/sumida-america-inc/0630CDMCCDS-3R3MC/5812457
Loop Compensation Resistor and Capacitor
With 4.7uF cap output, and around 10kHz bandwidth (this is 1/100 of the switching frequency but the stack exchange https://electronics.stackexchange.com/questions/479305/low-cost-buck-smps-with-ic-aoz1284-5v-x-4a-values-for-comp-pin answer verified with sims that around 10kHz is a more realistic achievement (Need to get this verified)
Rc is about 2050 ohms so 2k resistor should work
RL is the load resistor value, load resistance = Vout/Iout = 5/4 = 1.25Ohms
CC is around 2.9375nF so a 3nF cap
Others
Enable Pin is high between 1.2V and 5V so 3.3V from the AFE IC will be supplied to this pin (filter cap needed??)
Soft start Pin: “A soft start process begins when the input voltage rises to 3V and voltage on EN pin is HIGH. In soft start process, a 2.5µA internal current source charges the external capacitor at SS. As the SS capacitor is charged, the voltage at SS rises. The SS voltage clamps the reference voltage of the error amplifier, therefore output voltage rising time follows the SS pin voltage. With the slow ramping up output voltage, the inrush current can be prevented. Minimum external soft-start capacitor 850pF is required, and the corresponding soft-start time is about 200µs.”
will be using 10nF cap, was available on the WARG component library (relatively similar value)
Schottky Diode:
To reduce the losses due to the forward voltage drop and recovery of diode, Schottky diode is recommended to use. The maximum reverse voltage rating of the chosen Schottky diode should be greater than the maximum input voltage, and the current rating should be greater than the maximum load current.
Using the SK54A-LTP Diode which is already in the warg component library
rated for 40V @ 5A which should be enough for 26Vin and 4A current out
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