Three sets of UVLO thresholds are available with turnon and turnoff thresholds of: (14.5 V and 9 V), (8.4 V and 7.6 V), and (7 V and 6.6 V) respectively. The first set is primarily intended for off-line and 48-V distributed power applications, where the wider hysteresis allows for lower frequency operation and longer soft-starting time of the converter. The second group of UVLO options is ideal for high frequency DC-DC converters typically running from a 12-VDC input. The third, and newest, set has been added to address battery powered and portable applications. Table 8-2 shows the maximum duty cycle and UVLO thresholds by device.

During UVLO thedevicedraws less than 100 µA of supply current. Once crossing the turnon threshold thedevicesupply current increases to a maximum of 3 mA, typically 2.3 mA. This low start-up current allows the power supply designer to optimize the selection of the startup resistor value to provide a more efficient design. In applications where low component cost overrides maximum efficiency, the low run current of 2.3 mA (typical) allows the control device to run directly through the single resistor to (+) rail, rather than requiring a bootstrap winding on the power transformer, along with a rectifier. The start and run resistor for this case must also pass enough current to allow driving the primary switching MOSFET, which may be a few milliamps in small devices.

Figure 8-2 UVLO ON and OFF Profile