Copyright © 2016

Microsemi

Page 22

Rev. 1.4, July 2017

CPG

– PoE BU

One Enterprise Aliso Viejo, CA 92656 USA

PD70211

PD Controller with Switching Regulator for

AF/AT/UPOE/HDBaseT/4-pair PoE Applications

effect the hysteresis resistor is in parallel to the lower resistor RLOWER. When the voltage on the VINS pin rises

above 1.2V, the UVLO comparator flips and the hysteresis resistor appears connected to 5V (output of the UVLO

comparator). The equivalent configurations are shown in Figure 5. After solving the equations, the following

example indicates the set thresholds. The values are as used in Figure 3.



















UPPER

LOWER

HYST

LOWER

HYST

LOWER

HYST

LOWER _EQUIV

LOWER

HYST

R

270k; R

8.66k; R

270k

Part 1: (VINS less than 1.2V)

Equivalent lower resistor is a parallel combination of R

and R

RR

8.66k 270k

R

8.391

R

R

8.66k 270k















UPPER

LOWER _EQUIV

UVLO_ UP

LOWER _EQUIV

k

The rising voltage threshold is

RR

270k 8.391k

V

VREF

1.2V

39.8V

R

8.391k































UPPER

UPPER

UVLO_DN

LOWER

HYST

Part 2: (VINS greater than 1.2V)

RR

V

VREF

VDD VREF

VREF

RR

270k

270k

1.2V

3.8V

1.2 34.8V

8.66k

270k

So with the selected resistors, we get a rising threshold of 39.8V, and a falling threshold of 34.8V.

VIN

RUPPER

RLOWER

RHYST

VVINS < 1.2V

Rising Input

VIN

RUPPER

RLOWER

RHYST

VVINS > 1.2V

Falling Input

5V

Figure 5: Equivalent Diagrams for UVLO and Hysteresis