Datasheet

V
IN
RCLASS
AUX
V
EE
V
CC
OUT
RT
FB
CS
COMP
SS
RTN
ARTN
+PoE
+AUX
- AUX
UVLO
UVLORTN
LM
5071
0.1
PF
100 k:
-PoE
+V
OUT
100 k:
LM5071
www.ti.com SNVS409E –NOVEMBER 2005–REVISED APRIL 2013
Figure 14. Simplified Schematic Showing Auxiliary Implementation
Power Supply Operation
Once the UVLO threshold has been satisfied, the interface controller of the LM5071 will charge up the SMPS
input capacitor through the internal power MOSFET. This load capacitance provides input filtering for the power
converter section and must be at least 5uF per the IEEE 802.3af specification. To accomplish the charging in a
controlled manner, the power MOSFET is current limited to 100mA.
The SMPS controller will not initiate operation until the load capacitor is completely charged. The power
sequencing between the interface circuitry and the SMPS controller occurs automatically within the LM5071.
Detection circuitry monitors the RTN pin to detect interface startup completion. When the RTN pin potential drops
below 1.5V with respect to V
EE
, the V
CC
regulator of the SMPS controller is enabled. The soft-start function is
enabled once the V
CC
regulator achieves minimum operating voltage. The inrush current limit only applies to the
initial charging phase. The interface power MOSFET current limit will revert to the default protection current limit
of 390mA once the SMPS is powered up and the soft-start pin sequence begins.
High Voltage Start-up Regulator
The LM5071 contains an internal high voltage startup regulator that allows the input pin (V
IN
) to be connected
directly to line voltages as high as 60V. The regulator output is internally current limited to 15mA. The
recommended capacitance range for the V
CC
regulator output is 0.1uF to 10uF. When the voltage on the V
CC
pin
reaches the regulation point of 7.8V, the controller output is enabled. The controller will remain enabled until V
CC
falls below 6.25V.
In typical applications, a transformer auxiliary winding is diode connected to the V
CC
pin. This winding should
raise the V
CC
voltage above 8.1V to shut off the internal startup regulator. Though not required, powering V
CC
from an auxiliary winding improves conversion efficiency while reducing the power dissipated in the controller.
The external V
CC
capacitor must be selected such that the capacitor maintains the V
CC
voltage greater than the
V
CC
UVLO falling threshold (6.25V) during the initial start-up. During a fault condition when the converter auxiliary
winding is inactive, external current draw on the V
CC
line should be limited such that the power dissipated in the
start-up regulator does not exceed the maximum power dissipation capability of the LM5071 package.
If the V
CC
auxiliary winding is used with a low voltage auxiliary supply (wall transformer), the VCC pin could back
feed through the LM5071 to the VIN pin. A diode from VCC to VIN should be used to clamp the VCC pin and
prevent this internal back feed. The winding voltage will remain the same and extra power will be dissipated in
the series resistor. Also, note that when using a very low voltage auxiliary supply (<14V), a diode from the AUX
supply to the VCC pin should be used to ensure VCC startup.
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