LM5071 datasheet(14/18 Pages) NSC

14 / 18 page

Gate Driver and Maximum Duty

Cycle Limit (Continued)

plished with an internal toggle flip-flop which ensures an

accurate duty cycle limit. The internal oscillator frequency of

the LM5071-50 is therefore twice the operating frequency of

the PWM controller (OUT pin).

The 80% maximum duty cycle limit of the LM5071-80 is

determined by the internal oscillator and varies more than

the 50% limit of the LM5071-50. For the LM5071-80, the

internal oscillator frequency and the operational frequency of

the PWM controller are equal.

Thermal Protection

Internal thermal shutdown circuitry is provided to protect the

integrated circuit in the event the maximum junction tem-

perature is exceeded. This feature prevents catastrophic

failures from accidental device overheating. When activated,

typically at 165 degrees Celsius, the controller is forced into

a low power standby state, disabling the output driver, bias

regulator, main interface pass MOSFET, and classification

regulator if enabled. After the temperature is reduced (typical

hysteresis = 25˚C ) the V

CC regulator will be enabled and a

softstart sequence initiated.

Thermal shutdown is not enabled during auxiliary power

operation as the power MOSFET is not running any current

and should not experience an over-temperature condition. If

the drain of the MOSFET exceeds 2.5V with respect to VEE

(internal Power Good de-assertion), PoE UVLO becomes

de-asserted (insertion of PoE or other 48V supply), or the

auxiliary power is removed, thermal limit will be re-enabled

immediately.

LM5071 Datasheet(PDF) 14 Page - National Semiconductor (TI)