The FRS pin programs the (free-running) oscillator frequency, and may also be used to synchronize the TPS23753A converter to a higher frequency. The internal oscillator sets the maximum duty cycle and controls the current-compensation ramp circuit, making the ramp height independent of frequency. R_FRS must be selected per Equation 5.

Equation 5.

The TPS23753A may be synchronized to an external clock to eliminate beat frequencies from a sampled system, or to place emission spectrum away from an RF input frequency. Synchronization may be accomplished by applying a short pulse ( > 25 ns) of magnitude V_SYNC to FRS as shown in Figure 8-7. R_FRS must be chosen so that the maximum free-running frequency is just below the desired synchronization frequency. The synchronization pulse terminates the potential ON-time period, and the OFF-time period does not begin until the pulse terminates. A short pulse is preferred to avoid reducing the potential ON-time.

Figure 8-7 shows examples of nonisolated and transformer-coupled synchronization circuits RT reduces noise susceptibility for the isolation transformer implementation. The FRS node must be protected from noise because it is high impedance.

Figure 8-7 Synchronization