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Component Sourcing
Buck:
Filter for voltage regulators rated between 30V-40V max input voltage (larger than expected 26V), 4A output current, and 5V output voltage
3V to 36V input voltage range
up to 95% efficiency
4A continuous output current
adjustable switching frequency
adjustable output voltage (0.8V to 30V)
AOZ1284PI Buck Design:
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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.
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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
Component Calculations
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Input Capacitor
From datasheet, at Vin = 24V, voltage ripple is 1V/vin??
Output Capacitor
At 24Vin, the datasheet specifies that output ripple is around 50mV/div?? What does this mean??
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Inductor Selection (Section 3 of the TI document)
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
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
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
Cell Monitor IC
https://www.digikey.ca/en/products/filter/power-management-pmic/battery-management/713
https://www.digikey.ca/en/products/detail/analog-devices-inc/LTC6810IG-2-3ZZPBF/9658988
https://www.digikey.ca/en/products/detail/rochester-electronics-llc/AD7280WBSTZ-RL/12102176
3-6s cell monitoring
no ADC (connect to MCU and use that one)
Current Sense
https://www.ti.com/lit/an/slua707/slua707.pdf?ts=1721224314354
AFE Design:
Does D23 need to be implemented if we are using an XT90 Connector?STM32
Using the STM32L431KCU6 (the same one from Meghan’s CAN adapter circuit)
Some thoughts:
Still not exactly sure how the cell monitoring should be handled, a BMS chip is kinda what I’m thinking or at least some sort of cell monitoring device. An issue I have right now is component sourcing since digikey’s search is yielding some fraudulent results (I think links 1 & 2 for the BMS) since most of the ones that say they can support up to 6 cells (from analog devices) really only support 2 Li-ion (I’m assuming this is the type of battery WARG uses).
The LT6810 chip seems really ideal but it operates SPI/I2C and is the master node in this case (is this alright?!?! I would expect this kind of peripheral to be the slave node in this case)
Most cell monitoring ICs are rated for more than 6 cells (usually 8-12) so I don’t think that is a huge problem that some pins won’t be used (redundant not optimal :0), almost none of them have a CAN bus serial protocol so I’ll need to use an IC to convert the signals most likely
Also, in a perfect world I would use the same IC to monitor current and cell voltage but most chips I have found only operate on one of the functionalities
Using two separate IC’s (one for BMS one for current), how does data transmission work if both are sending data on the line?
Optimal IC I think would be the BQ7692, might be wrong here but I think it has current sense capabilities
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