SNVS480J January 2007 – July 2020 LM5022
PRODUCTION DATA.
The current sensing resistor, R_{SNS}, is used for steady-state regulation of the inductor current and to sense overcurrent conditions. The slope compensation resistor is used to ensure control loop stability, and both resistors affect the current limit threshold. The R_{SNS} value selected must be low enough to keep the power dissipation to a minimum, yet high enough to provide good signal-to-noise ratio for the current sensing circuitry. R_{SNS} and R_{S2} must be set so that the current limit comparator, with a threshold of 0.5 V, trips before the sensed current exceeds the peak current rating of the inductor, without limiting the output power in steady state.
For this example, the peak current at V_{IN(MIN)} is 2.5 A, while the inductor itself is rated to 3.2 A. The threshold for current limit, I_{LIM}, is set slightly between these two values to account for tolerance of the circuit components, at a level of 3 A. The required resistor calculation must take both the switch current through R_{SNS} and the compensation ramp current flowing through the internal 2-kΩ R_{S1} and R_{S2} resistors into account. R_{SNS} must be selected first because it is a power resistor with more limited selection. Equation 36 and Equation 37 must be evaluated at V_{IN(MIN)} when duty cycle is highest.
where
The closest 5% value is 100 mΩ. Power dissipation in R_{SNS} can be estimated by calculating the average current. The worst-case average current through R_{SNS} occurs at minimum input voltage/maximum duty cycle and can be calculated with Equation 38 and Equation 39.
For this example, a 0.1 Ω ±1%, thick-film chip resistor in a 1210 case size rated to 0.5 W is used.
With R_{SNS} selected, R_{S2} can be determined using Equation 40 and Equation 41.
The closest 1% tolerance value is 3.57 kΩ.