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Exposed Pad (Vin Exposed pad - pin number on symbol??? is this ok in general for a vin pin to be an exposed pad?)
What should I use for switching frequency? From what I’ve seen online, higher switching frequency means I can use smaller inductors and capacitors but increases losses.
Going to use 1M switching frequency for reduced board space. Seems to be a typo/mistake in the datasheet as the 5000 should actually be 55000 or 52500, so in theory RF should be around 47.5 to 50 kOhms. I will be using a 47k resistor because Digikey and Yageo do not offer a 46.6k
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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
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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Input Capacitor
From datasheet, at Vin = 24V, voltage ripple is 1V/vin??Ideally <1% of Vin for ripple voltage so 0.01* (20V or 26V) ripple range: 0.2V to 0.26V
Cin should be between 3.75uF and 2.39uF, 3.3uF should be fine
Output Capacitor
At 24Vin, the datasheet specifies that output ripple is around 50mV/div?? What does this mean??From online, it seems around 1% of Vout ripple voltage is desirable (lower is better obv)
Since we’re using X5R and X7R capacitors I’m assuming ESR can be ignored???
Vout is 5V so 1% is 0.05V would be preferred
For ripple current 0.2,0.3,0.4 of Iout = 4A (0.8A, 1.2A, 1.6A → 2uF, 3uF, 4uF)
higher capacitance = less ripple voltage = better 😄
https://rfcafe.com/references/electrical/capacitor-values.htm
Standard cap values within range: 2.2uF, 3.3uF
I will be choosing 3.3uF (at 0.3Iout ripple current, this is around 0.9% of output voltage (45mV)
Should be rated for >4A and >5V
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Inductor Selection (Section 3 of the TI document)
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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
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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