Design Idea DI-102
®
DPA-Switch
Lowered Overvoltage
for Power over Ethernet (PoE)
Application
PoE/VoIP
Device
DPA423G
Power Output
-
Input Voltage
36-57 VDC
Output Voltage
-
Topology
-
Design Highlights
• Optimized overvoltage for PoE Powered Devices (PDʼs)
• Turn-off threshold 65 VDC and turn-on threshold
63 VDC
• Compliance to IEEE 802.3af standards over complete
voltage window ensures compatibility with power
sending equipment
Operation
This circuit allows the overvoltage shutdown threshold to be set
to approximately 63 VDC. The
DPA-Switch
detects the input
voltage via the current in L-pin resistor R
LS
. Above the OV-off
threshold (135
µA),
the
DPA-Switch
is disabled and below the
OV-on threshold (131
µA),
the
DPA-Switch
becomes operational
again. At start-up, transistor Q2 is pulled low (off) via resistor R5,
so as not to interfere with the under-voltage detection threshold.
Transistor Q2 is pulled high (on) via resistor R4 and will turn
on once the input voltage comfortably exceeds the under-
voltage turn-on level, at the defined threshold voltage (V
IN(th)
=
60 VDC). When turned-on, transistor Q2 connects the Control
pin (C) voltage (V
C
) to the L-pin via R6, thus adding a fixed
current (approximately 37
µA)
to the L-pin. This additional
current lowers the OV on and off voltage thresholds.
Wide Hysteresis Overvoltage
The default under-voltage and overvoltage shutdown
thresholds of the
DPA-Switch
are programmed with a single
resistor (R
LS
) connected from the positive input voltage to the
L-pin. The default overvoltage and under-voltage thresholds
have a fixed ratio (approximately 2.7:1).
The operating voltage range for PoE systems is 36 VDC to
57 VDC, a much smaller ratio. Overvoltage Shutdown (OVSD)
on the PD allows protection against possible system faults,
giving the design an increased level of robustness. This can
be achieved by adding a simple discrete circuit.
ON
I
UV(ON)
I
OV(OFF)
R
LS
Only
+
OFF
0
25
47
µA
R
LS
619 kΩ
1%
Q2
D
L
CONTROL
50
75
100
131
µA
125
150
135
µA
I
L
R4
ON
DC Input
Voltage
R6
C
R5
R
LS
with
Q2 Circuit
37
µA
OFF
0
PI-4065-083105
-
S
DPA-Switch
25
50
75
100
125
150
I
L
Figure 1. DPA-Switch with Lower OVSD (R
1
= 10 MΩ,
R
2
= 560 kΩ, R
3
= 82 kΩ).
L Pin Current (µA)
PI-4067-090105
Figure 2. L-Pin Current without/with Wide OVSD Circuit.
DI-102
www.powerint.com
August 2005
POWERINT [ POWER INTEGRATIONS, INC. ]
