Select the series resistor such that sufficient ripple is injected at the feedback node (FB). The ripple voltage produced by RESR is proportional to the inductor ripple current. Therefore, select RESR based on the lowest inductor ripple current occurring at minimum input voltage. Calculate RESR usingEquation 17.
With VO = 5 V, VREF = 2 V and ΔIL, min = 180 mA (at VIN, min= 10 V) as calculated in Equation 14, Equation 17 requires an RESR greater than or equal to 0.35 Ω. Selecting R7 (RESR) = 0.47 Ω results in approximately 150 mV of maximum output voltage ripple at VIN,max. For applications requiring lower output voltage ripple, use Type II or Type III ripple injection circuits as described inRipple Configuration.