Design Idea DI-101

Wide Hysteresis

Design Highlights

• High hysteresis under-voltage lockout for Power over

Ethernet (PoE) Powered Devices (PDs)

• Turn-on threshold of 42 VDC and turn-off threshold of

34 VDC

• CompliancetoPoEstandard(IEEE802.3af)overcomplete

voltagewindowensurescompatibilitywithPSEequipment.

Tested by University of New Hampshire Interoperability

Consortium (UNH-IOC)*

Wide Hysteresis Under-Voltage

Thedefaultunder-voltagelockoutandovervoltageshutdown

thresholds of the DPA-Switch are programmed with a single

resistor (R

to the L-pin. The default overvoltage and under-voltage

thresholds have a fixed ratio (ratio approximately 2.7:1).

In a PoE system, the Power Sourcing Equipment (PSE)

provides a minimum operating voltage of 44 VDC.

However, the PoE specification allows CAT-5 (Ethernet)

cable lengths up to 100 meters/ 300 ft (with up to

20 Ω of cable impedance). At peak operating current

(350 mA), the voltage at the PD can drop to approximately

37 VDC (350 mA × 20 Ω). A minimum of 7 VDC under-

voltage hysteresis is required to accommodate the cable drop

voltage and prevent nuisance lockouts from occurring.

Operation

This circuit takes advantage of accurate DPA-Switch L-pin

current sensing as the basis for a new composite threshold,

programmed for 42 VDC turn-on and 34 VDC turn-off.

The DPA-Switch detects the input voltage via current in the

L-pin resistor R

DPA-Switch

begins switching and below an UV-off threshold

(47 µA), the DPA-Switch is disabled. Transistor Q1 is turned

on via bias resistor R2, causing a voltage drop across R3 and

subtracting a fixed current of approximately 10 µA(moving the

effective threshold to 60 µA) from the L-pin. When the power

supply becomes operational, the bias voltage pulls up via R1,

turning off Q1, cutting off the R3 current and returning to the

default UV-off threshold (47 µA).

www.powerint.com

Figure 1. DPA-Switch with Wide Hysteresis UVLO.

DI-101

®

November 2005

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

PoE/VoIP

DPA423G

-

34-57 VDC

-

-

Figure 2. L-Pin Current without/with Wide UVLO Circuit.

25

50

75

100

125

150

0

ON

OFF

PI-4068-111005

L Pin Current (

µA)

131

µA

135

µA

47

µA

10

µA

60

µA

25

50

75

100

125

150

0

ON

OFF

Q1 Circuit

PI-4064-111005

DC Input

Voltage

+

-

D

S

C

CONTROL

L

649 k

Ω

1%

+

-

R1

10 k

Ω

R2

10 k

Ω

R3

174 k

Ω

1%

Bias

Voltage

DPA-Switch

Q1

MMST3906

*UNH-IOC test reports are available on the PI website www.powerint.com/poe