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$ R_{IMON} (\Omega) = \frac{V_{IMON}(\mu V)}{I_{LOAD} (A) * G_{IMON}(\mu A/A)} = \frac{V_{IMON}(V)}{I_{LOAD} (A) * G_{IMON}(\mu A/A)*10^610^{-6}}$ |
Our ADC can read a maximum value of 32.3 V (most likely, need to confirm5V (based on voltage reference provided to the ADC), so V_IMON = 32.3 5 V.
Table 8-2 of the eFuse datasheet recommends using a lower V_IMON to “ensure the IMON pin internal amplifier has sufficient headroom to operate linearly.” For V_IN > 5V, it recommends a maximum V_IMON of 3.3V, which is met with our selection.
The maximum ILOAD of the eFuse is 18 A. Although most, if not all, of our outputs will not utilize that full quantity, we will design for the full range of possible values to avoid having to resolder IMON resistors in the future. This will reduce our precision when reading the current as a tradeoff.
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$R_{IMON}(\Omega) = \frac{32.35*10^6}{18 * 243} = 754571.458559 \Omega \rightarrow 732549 \Omega $ |
To measure the full range of currents, we need a 754571.458 559 Ω resistor. Finding a close-by standard resistor size, we moved down to 732 549 Ω ensure that in the event of an overcurrent event greater than 18A, we’d still be able to measure it to a degree without exceeding the ADC’s limits (or even worse, damaging the ADC IC). With 549 Ω, the largest current we can measure is 18.740 A nominal (with 1% resistor, 18.554 - 18.929 A).
Read the voltage drop across the resistor to find the current through the eFuse. Add a low-pass filter to smooth the results and prevent aliasing in the ADC conversion.
Result: Install a 732 549 Ω 1% resistor from IMON to ground.
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Gate Resistance (for external FET)
Picked from reference design.
External FET: CSD17573Q5B (
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The TP25983 offers the ability to drive the gate of an external FET to block reverse current.
This MOSFET was picked because it is what’s used in TI’s development board for the eFuse we’re using. It wasn’t clear what requirements there were for external FETs on the eFuse datasheet so we’re using a MOSFET that is known to work. This also means we don’t have to decide the gate resistance ourselves.
We need to ensure whatever voltage the BGATE pin is at is sufficient for the MOSFET of choice so that the MOSFET stays in the linear region. However, the voltage BGATE works at is not clear. Page 35 of the eFuse datasheet states that “The dVdt capacitor is subjected to typically VIN + 4 V during startup”, but unsure about regular operation.
Digital POT IC: AD5258BRMZ1 (
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Another potential option could be the AD8400. It has a smaller package, more resistance positions, and a smaller package, but is significantly more expensive.
I2C Mux: PCA9544A
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First one that showed up when Googled for I2C mux. Seems to fit purpose.
Monitoring Parts Selection
SOM:
fwef
Analog MUX: ADS7953SBDBT (
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The TP25983 offers the ability to drive the gate of an external FET to block reverse current.
Monitoring Parts Selection
SOM:
fwef
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Same as what BPS is using.
Reference Voltage: MIC5317-2.5
Same as what BPS is using.
SPI I/O Expander: MCP23S17 (
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(I2C equivalent: MCP23017)
Offers both SPI and I2C variant.
Has interrupt pin(s).
Has addressing system in SPI which provides more flexibility.