SNVA406C May   2010  – January 2022 LM27402

 

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9.8 Current Limit Circuitry, Rs, Cs, Rs1, Rs5, Rtc

The current limit circuitry included on the LM27402 evaluation board sets the current limit at 24 A. Components Rs and Cs connect directly under the inductor pads and create an RC filter. The time constant of RsCs should match the time constant of the inductance and DCR of the inductor:

Equation 11. GUID-74A2349F-02D6-4C60-B477-BD1299B92F6C-low.gif

A typical range of capacitance used in the RsCs network is 100 nF to 1 µF. A 220-nF capacitor was chosen for the Cs filter capacitor resulting in an Rs resistor of:

Equation 12. GUID-68273F08-84F7-4390-92E4-90599BB66412-low.gif

A standard value resistor of 1.3 kΩ was selected for Rs. The current limit level is set through a resistor from CS- to the VOUT pad of the inductor. The LM27402 evaluation board is set to current limit at 24-A IOUT. The maximum inductor current is IOUT + ΔIL / 2 = 24 + 6.4 / 2 = 27.2 A. The next equation describes the current limit resistor calculation:

Equation 13. GUID-AF695511-1F6A-4801-9715-D147DAC5D8DC-low.gif

Copper resistance changes by about 3900 ppm/°C and can cause a significant error in the current limit setpoint. The LM27402 evaluation board is equipped with a 5.6-kΩ positive temperature coefficient resistor (Rtc) to compensate the effects of copper resistance and a 750-Ω resistor Rs3 in series with Rtc to approximately provide the 6.36 kΩ needed for RSET. Rtc was chosen to be a Vishay TFPT1206L5601F 5.6-kΩ resistor which has a temperature coefficient of 4110 ppm/°C. An optional 6.34-kΩ resistor (Rs1) was placed between CS+ and the RsCs filter to mirror the impedance of the CS- pin in addition to a 100-pF capacitor placed between CS+ and CS- near the IC to reduce the effects of noise.

The internal 10-µA current source is powered from VIN. If the voltage between VIN and CS- is below 1 V, the current source will supply less than 10 µA. If this happens, the common mode voltage of the current sense comparator inputs (CS+ and CS-) can be decreased to ensure 10 µA of current. Extra resistor pads (Rs1, Rs2, Rs4) are included in the LM27402 evaluation board to lower the common mode voltage. Please refer to the AN-2060 LM27402 Current Limit Application Circuits application report for design guidelines to adjust the common mode voltage of the current sense comparator.