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LM5071 Datasheet(PDF) 11 Page - National Semiconductor (TI) |
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LM5071 Datasheet(HTML) 11 Page - National Semiconductor (TI) |
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11 / 18 page  Classification (Continued) TABLE 2. Classification Levels and Required External Resistors Class PMIN PMAX ICLASS (MIN) ICLASS (MAX) RCLASS 0 0.44W 12.95W 0mA 4mA Open 1 0.44W 3.84W 9mA 12mA 150 Ω 2 3.84W 6.49W 17mA 20mA 82.5 Ω 3 6.49W 12.95W 26mA 30mA 54.9 Ω 4 Reserved Reserved 36mA 44mA 38.3 Ω Undervoltage Lockout (UVLO) The IEEE 802.3af specification states that the PSE will supply power to the PD within 400ms after completion of detection. The LM5071 contains a programmable line Under Voltage Lock Out (UVLO) circuit. The first resistor should be connected between the V IN to UVLO pins; the bottom resis- tor in the divider should be connected between the UVLO and UVLORTN pins. The divider must be designed such that the voltage at the UVLO pin equals 2.0V when V IN reaches the desired mini- mum operating level. If the UVLO threshold is not met, the interface control and SMPS control will remain in standby. UVLO hysteresis is accomplished with an internal 10uA current source that is switched on and off into the impedance of the UVLO set point divider. When the UVLO threshold is exceeded, the current source is activated to instantly raise the voltage at the UVLO pin. When the UVLO pin voltage falls below the 2.00V threshold, the current source is turned off, causing the voltage at the UVLO pin to fall. The LM5071 UVLO thresholds cannot be programmed lower than 25V, the AUX pin should be used to force UVLO release below 25V. There are many additional uses for the UVLO pin. The UVLO function can also be used to implement a remote enable / disable function. Pulling the UVLO pin down below the UVLO threshold disables the interface and SMPS controller unless forced on via AUX pin operation. AUX Pin Operation The AUX pin can be used to force operation (UVLO release) of the interface and switching regulator at any input voltage above 9.5V. This is especially useful for auxiliary input (wall transformer) input voltages. The pin has a 2.5V threshold (0.5V hysteresis) and an input impedance of approximately 350k Ω. The input resistor provides a defined pull down impedance if the pin is left open by the user. An external pull down resistor should be used to provide additional noise immunity. The resultant pin voltage from the external resistor divider should be well above the 2.5V threshold to ensure proper auxiliary operation. See Figure 4 for an example of a simple yet robust auxiliary configuration. 20168422 FIGURE 4. Simplified Schematic Showing Auxiliary Implementation www.national.com 11 |
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