7.3.6 On-Time Timer, Shutdown

The on-time of the LMR24220 main MOSFET is determined by the resistor R_ON and the input voltage V_IN. It is calculated as follows:

Equation 4.

The inverse relationship of t_on and V_IN gives a nearly constant frequency as V_IN is varied. Select R_ON so that the on-time at maximum V_IN is greater than 150 ns. The on-timer has a limiter to ensure a minimum of 150 ns for t_on. This limits the maximum operating frequency, which is governed by Equation 5:

Equation 5.

The LMR24220 can be remotely shutdown by pulling the voltage of the EN pin below 1 V. In this shutdown mode, the SS pin is internally grounded, the on-timer is disabled, and bias currents are reduced. Releasing the EN pin allows normal operation to resume because the EN pin is internally pulled up.

Figure 17. Shutdown Implementation