LM5071 datasheet(13/24 Pages) TI

13 / 24 page

VIN

RCLASS

AUX

VEE

VCC

OUT

COMP

RTN

ARTN

+PoE

+AUX

- AUX

UVLO

UVLORTN

LM 5071

0.1 PF

100 k:

-PoE

+VOUT

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 VEE, the VCC regulator of the SMPS controller is enabled. The soft-start function is

enabled once the VCC 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 (VIN) 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 VCC regulator output is 0.1uF to 10uF. When the voltage on the VCC pin

reaches the regulation point of 7.8V, the controller output is enabled. The controller will remain enabled until VCC

falls below 6.25V.

In typical applications, a transformer auxiliary winding is diode connected to the VCC pin. This winding should

raise the VCC voltage above 8.1V to shut off the internal startup regulator. Though not required, powering VCC

from an auxiliary winding improves conversion efficiency while reducing the power dissipated in the controller.

The external VCC capacitor must be selected such that the capacitor maintains the VCC voltage greater than the

VCC 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 VCC 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 VCC 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.

Submit Documentation Feedback

Product Folder Links: LM5071

LM5071 Datasheet(PDF) 13 Page - Texas Instruments

LM5071 Datasheet(HTML) 13 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL