Design Idea DI-101
DPA-Switch® Under-Voltage with
Wide Hysteresis
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
PoE/VoIP
DPA423G
-
34-57 VDC
-
-
Design Highlights
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.
• 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
Operation
• CompliancetoPoEstandard(IEEE802.3af)overcomplete
voltagewindowensurescompatibilitywithPSEequipment.
Tested by University of New Hampshire Interoperability
Consortium (UNH-IOC)*
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 RLS. Above an UV-on threshold (50 µA), the
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).
Wide Hysteresis Under-Voltage
Thedefaultunder-voltagelockoutandovervoltageshutdown
thresholds of the DPA-Switch are programmed with a single
resistor (RLS) connected from the positive input voltage
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
ON
IUV(ON)
IOV(OFF)
RLS Only
+
OFF
ON
RLS
649 kΩ
0
25
50
75
100
131 µA
125
150
IL
+
1%
R3
47 µA
135 µA
R1
174 kΩ
10 kΩ
1%
DC Input
Voltage
Bias
Voltage
Q1
MMST3906
10 µA
D
S
L
CONTROL
RLS with
Q1 Circuit
C
DPA-Switch
R2
10 kΩ
-
-
OFF
PI-4064-111005
0
25
50
60 µA
75
100
125
150
IL
Figure 1. DPA-Switch with Wide Hysteresis UVLO.
PI-4068-111005
L Pin Current (µA)
Figure 2. L-Pin Current without/with Wide UVLO Circuit.
*UNH-IOC test reports are available on the PI website www.powerint.com/poe
DI-101
November 2005
www.powerint.com