The on-time for the LM5008A is determined by the RT resistor and is inversely proportional to the input voltage (VIN), resulting in an almost constant frequency as Vin is varied over its range. The on-time equation for the LM5008A is Equation 8.
RT must be selected for a minimum on-time (at maximum VIN) greater than 400 ns, for correct current limit operation. This requirement limits the maximum frequency for each application, depending on VIN and VOUT.
The LM5008A can be remotely disabled by taking the RT/SD pin to ground. See Figure 53. The voltage at the RT/SD pin is between 1.5 V and 3 V, depending on VIN and the value of the RT resistor.