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6.3.12.1 Shunt Current Sensing

Shunt current sensing illustrates inductor current sensing using a shunt resistor. This configuration continuously

monitors the inductor current to provide accurate overcurrent protection across the operating temperature range.

For optimal current sense accuracy and overcurrent protection, use a low inductance ±1% tolerance shunt

resistor between the inductor and the output, with a Kelvin connection to the LM25139 current sense amplifier.

If the peak voltage signal sensed from ISNS to VOUT exceeds the current limit threshold of 60mV, the current

limit comparator immediately terminates the HO output for cycle-by-cycle current limiting. Use Equation 10 to

calculate the shunt resistance.

RS= VCSTH

IOUTCL + ΔIL2

(10)

where

• VCS-TH is current sense threshold of 60mV.

• IOUT(CL) is the overcurrent setpoint that is set higher than the maximum load current to avoid tripping the

overcurrent comparator during load transients.

• ΔIL is the peak-to-peak inductor ripple current.

VOUT

Current sense

amplifier

COUT

ISNS

VIN

CS gain = 10

Figure 6-5. Shunt Current Sensing Implementation

The soft-start voltage is clamped 95mV above FB during an overcurrent condition and after soft-start is complete

(8ms timer). 16 overcurrent events must occur before the SS clamp is enabled. This amount makes sure that SS

can be pulled low during brief overcurrent events, preventing output voltage overshoot during recovery.

6.3.12.2 Inductor DCR Current Sensing

For high-power applications that do not require accurate current-limit protection, inductor DCR current sensing

is preferable. This technique provides lossless and continuous monitoring of the inductor current using an RC

sense network in parallel with the inductor. Select an inductor with a low DCR tolerance to achieve a typical

current limit accuracy within the range of 10% to 15% at room temperature. Components RCS and CCS in Figure

6-6 create a low-pass filter across the inductor to enable differential sensing of the voltage across the inductor

DCR.

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LM25139

SLVSJ80 – OCTOBER 2025

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