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• Our ADC can read a maximum value of 3.3 V (most likely, need to confirm), so V_IMON = 3.3 V. Although,

  • Table 8-2 of the eFuse datasheet recommends

  leaving

  • 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.

$R_{IMON}(\Omega) = \frac{3.3*10^6}{18 * 243} = 754.458 \Omega \rightarrow 732 \Omega $

To measure the full range of currents, we need a 754.458 Ω resistor. Finding a close-by standard resistor size, we moved down to 732 Ω ensure that in the event of an overcurrent event greater than 18A, we’d still be able to measure it without exceeding the ADC’s limits (or even worse, damaging the ADC IC).

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 Ω 1% resistor from IMON to ground.

Retry Delay

The delay before retry attempts after a fault is determined by…

$t_{RETRY\_DLY} (\mu s) = \frac{128 * (C_{RETRY\_DLY} (pF) + 4 pF) * V_{RETRY\_DLY\_HYS} (V)}{I_{RETRY\_DLY} (\mu A)}$

Solving for the capacitance at the RETRY_DLY pin…

$C_{RETRY\_DLY} (pF) = \frac{t_{RETRY\_DLY} (\mu s)*I_{RETRY\_DLY} (\mu A)}{128*V_{RETRY\_DLY\_HYS} (V)} - 4pF$

(or just use the table at the end of the Retry # of Attempts section for some common values.)

Note: the Retry # of Attempts capacitance depends on this value. If a change is made here, NRETRY needs to be reevaluated.Retry Delay

Result: We will implement a 915 ms delay, resulting in a 22 nF capacitor to ground at the RETRY_DLY pin.

Retry # of Attempts

The capacitance value for number of retry attempts depends on what capacitance you picked for retry delay.

$N_{RETRY} = \frac{4 * I_{RETRY\_DLY}(\mu A) * C_{NRETRY}(pF)}{I_{NRETRY}(\mu A) * (C_{RETRY\_DLY}(pF) + 4pF)}$

Solving for the amount of capacitance for a certain N retries…

$C_{NRETRY}(pF)= \frac{N_{RETRY}*I_{NRETRY}(\mu A) * (C_{RETRY\_DLY}(pF) + 4pF)}{4*I_{RETRY\_DLY}(\mu A)}$ 

Note that N can only consist of the set of values {(0), 4, 16, 64, 256, 1024, inf}. Any value not equal to these will be rounded up.

For an infinite amount of attempts, short NRETRY to ground. For no auto-retry (latch off), short RETRY_DLY to ground (NRETRY does not matter).

For a set of common RETRY_DLY and NRETRY values…

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Result: We will implement 4 retry attempts. With the previously selected 915 ms delay between each, we will place no capacitor between NRETRY and ground (open).

Slew Rate

The integrated output slew rate helps control large inrush current. The formula for determining the slew rate of a particular load capacitance is…

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