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JAJSUV0 June 2024 LM5171

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7.2.1.2.12 Channel Current Monitor (IMONx)

For best current monitor accuracy, choose IMONx resistor that the maximum operating voltage on the IMONx pin is less than 3V.

Considering two phase current monitoring with maximum 50A for each channel, and make sure IMONx voltage no more than 3V, R_IMONx can be find as:

Equation 105. R_IMONx = 10kΩ

Choose C_IMONx considering delay and voltage ripple. Here we select:

Equation 106. C_IMONx = 10nF

Then the delay of the monitor is determined by the following time constant:

Equation 107.

τ_{I M O N x} = R_{I M O N x} \times C_{I M O N x} = 10 k Ω \times 10 n s = 100 μ s

At full load, the DC component of the monitor voltage is:

Equation 108.

V_{I M O N x} = 2 \times (\frac{I_{m a x} \times R_{C S}}{500 Ω} + 50 μ A) \times R_{I M O N x} = 2 \times (\frac{30 A \times 1 m Ω}{500 Ω} + 50 μ A) \times 10 k Ω = 2.2 V

Considering the inductor ripple current, according to Equation 6, the IOUT peak to peak ripple current is:

Equation 109.

Δ I M O N x = \frac{I_{p k - p k} \times R_{C S}}{500 Ω} = \frac{23.8 A \times 1 m Ω}{500 Ω} = 47.6 μ A

The RC filter corner frequency is thus given by:

Equation 110.

f_{I M O N x} = \frac{1}{2 π \times R_{I M O N x} \times C_{I M O N x}} = \frac{1}{6.28 \times 10 k Ω \times 10 n F} = 1.59 k H z

The resulting peak-to-peak monitor ripple voltage is approximately determined by:

Equation 111.

Δ V_{I M O N x} = Δ I M O N x \times (R_{I M O N x}‖ \frac{1}{2 i \times π \times F_{S W} \times C_{I M O N x}}) = 47.6 μ A \times (10 k Ω‖ \frac{1}{2 i \times π \times 100 k H z \times 10 n F}) = 7.5 m V

The peak-to-peak monitor ripple voltage is approximately 0.34% of the full load DC monitor voltage. Increasing C_IMONx attenuates the ripple voltage at the cost of higher monitor delay.

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7.2.1.2.12 Channel Current Monitor (IMONx)