4 Operation Notes

In order to operate the EVM using a single power supply, the Vin Select jumper (P1) must be in the default PVIN-VIN position shown in Figure 2-1. In this position, the PVin and Vin pins of the device are connected together. The UVLO threshold of the EVM is approximately 4 V with 0.15 V of hysteresis. The input voltage must be above the UVLO threshold in order for the device to start-up. After start-up, the minimum input voltage to the device must be at least 4.5 V or (VOUT + 0.7 V), whichever is greater. The maximum operating input voltage for the device is 17 V. Refer to the LMZ317xx data sheets for further information on the input voltage range, UVLO operation, and optional split power supply operation for operating with PVin as low as 2.95 V when using an external Vbias supply.

After application of the proper input voltage, the output voltage of the device will ramp to its final value in approximately 1.2 ms. If desired, this soft-start time can be increased by adding a capacitor to the SS/TR test point as described above. Refer to the LMZ317xx data sheets for further information on adjusting the soft-start time.

Table 2-1 lists the recommended switching frequencies for each of the VOUT selections. These recommendations cover operation over a wide range of input voltage and output load conditions. Several factors such as duty cycle, minimum on-time, minimum off-time, and current limit influence selection of the appropriate switching frequency. In some applications, other switching frequencies can be used for particular output voltages, depending on the above factors. Refer to the LMZ317xx data sheets for further information on switching frequency selection, including synchronization.

The EVM includes input and output capacitors to accommodate the entire range of input and output voltage conditions. The actual capacitance required will depend on the input and output voltage conditions of the particular application, along with the desired transient response. In most cases, the required output capacitance will be less than that supplied on the EVM. Refer to the LMZ317xx data sheets for further information on the minimum required I/O capacitance and transient response.

The LMZ317xx operates in pulse skip mode at light currents to improve light load efficiency (LLE mode). At output voltages of less than 1.5 V, the pulse skipping can cause the output to rise when there is no load to discharge the energy. A minimum load of 600 µA or less, depending on VOUT, is required to keep the output voltage within regulation. For the worst case condition of V_OUT = 0.6 V, a 1-kΩ resistor would provide a required minimum load of 600 µA. If the application requires an additional load to meet the minimum load requirement, the additional load can be connected external to the EVM or installed in the R16 position on the underside of the EVM. Refer to the LMZ317xx data sheets for further information on LLE mode and determining the required minimum load.