LM5071MT-50 NSC [National Semiconductor], LM5071MT-50 Datasheet - Page 11

• Manufacturer Part Number: LM5071MT-50
• Description: Power Over Ethernet PD Controller with Auxiliary Power Power Over Ethernet PD Controller with Auxiliary Power
• Manufacturer: NSC [National Semiconductor]

Classification
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
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
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
Class
TABLE 2. Classification Levels and Required External
0
1
2
3
4
Reserved
0.44W
0.44W
3.84W
6.49W
PMIN
Reserved
12.95W
12.95W
PMAX
3.84W
6.49W
Resistors
(Continued)
IN
FIGURE 4. Simplified Schematic Showing Auxiliary Implementation
to UVLO pins; the bottom resis-
IN
ICLASS
17mA
26mA
36mA
(MIN)
0mA
9mA
reaches the desired mini-
ICLASS
(MAX)
12mA
20mA
30mA
44mA
4mA
RCLASS
82.5Ω
54.9Ω
38.3Ω
Open
150Ω
11
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
350kΩ. 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
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