7.2.6 ON-Time Timer, Shutdown

The on-time of the LM3103 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. R_ON should be selected such that the on-time at maximum V_IN is greater than 100 ns. The on-timer has a limiter to ensure a minimum of 100 ns for t_on. This limits the maximum operating frequency, which is governed by the following equation:

Equation 5.

The LM3103 can be remotely shut down by pulling the voltage of the EN pin below 1.6 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 19. Shutdown Implementation