In external synchronization mode, a resistor is connected between the RT pin and GND corresponding to the external clock frequency as indicated in Equation 4 and Figure 7-4. During this mode of operation, the SYNC1 pin connects to the input clock and the SYNC2 pin must be connected to either GND or VIN depending on whether it is desired to invert the clock SYNC1 receives. When SYNC2 is connected to GND, the clock provided on SYNC1 is inverted. When SYNC2 is connected to VIN, the input clock signal on SYNC1 does not get inverted. As a result, external synchronization mode can be used to connect 2, 3, or 4 devices in parallel using an external clock (at any frequency between 100 kHz and 1 MHz) as long as the clocks used for each device are in the proper out of phase configuration. If no external clock signal is detected for 20 µs, then the TPS7H4003-SEP transitions to its internal clock and a switching frequency that is determined by the value of the RT resistor. If no external clock is available, then the primary-secondary operation mode can also be used to connect devices in parallel.