Rainbow Electronics MAX5965B - Manual (Page 17)

MAX5965A/MAX5965B
High-Power, Quad, Monolithic, PSE Controllers
for Power over Ethernet
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A valid PD has a 25kΩ discovery signature characteristic
as specified in the IEEE 802.3af/at standard. Table 1
shows the IEEE 802.3af/at specification for a PSE detect-
ing a valid PD signature. See the
Typical Operating
Circuits and Figure 1a (Detection, Classification, and
Power-Up Port Sequence). The MAX5965A/MAX5965B
can probe and categorize different types of devices con-
nected to the port such as: a valid PD, an open circuit, a
low resistive load, a high resistive load, a high capacitive
load, a positive DC supply, or a negative DC supply.
During detection, the MAX5965A/MAX5965B keep the
external MOSFET off and force two probe voltages
through the DET_ input. The current through the DET_
input is measured as well as the voltage at OUT_. A
two-point slope measurement is used as specified by
the IEEE 802.3af standard to verify the device connect-
ed to the port. The MAX5965A/MAX5965B implement
appropriate settling times and a 100ms digital integra-
tion to reject 50Hz/60Hz power-line noise coupling.
An external diode, in series with the DET_ input, restricts
PD detection to the first quadrant as specified by the
IEEE 802.3af/at standard. To prevent damage to non-PD
devices, and to protect themselves from an output short
circuit, the MAX5965A/MAX5965B limit the current into
DET_ to less than 2mA maximum during PD detection.
In midspan mode, the MAX5965A/MAX5965B wait 2.2s
before attempting another detection cycle after every
failed detection. The first detection, however, happens
immediately after issuing the detection command.
High-Capacitance Detection
The CLC_EN bit in register R23h[5] enables the large
capacitor detection feature for legacy PD devices.
When CLC_EN = 1, the high-capacitance detection limit
is extended up to 150µF. CLC_EN = 0 is the default
condition for the normal capacitor size detection. See
Table 1 and the
Register Map and Description section.
Table 1. PSE PI Detection Modes Electrical Requirement
(Table 33-2 of the IEEE 802.3af Standard)
PARAMETER SYMBOL MIN MAX UNITS ADDITIONAL INFORMATION
Open-Circuit Voltage V OC — 30 V In detection mode only
Short-Circuit Current I SC — 5 mA In detection mode only
Valid Test Voltage V VALID 2.8 10 V
Voltage Difference
Between Test Points
ΔV
TEST 1 — V
Time Between Any Two
Test Points
t BP 2 — ms
This timing implies a 500Hz maximum probing
frequency
Slew Rate V SLEW 0.1 V/µs
Accept Signature
Resistance
R GOOD 19 26.5 k Ω
Reject Signature
Resistance
R BAD < 15 > 33 k Ω
Open-Circuit Resistance R OPEN 500 — k Ω
Accept Signature
Capacitance
C GOOD — 150 nF
Reject Signature
Capacitance
C BAD 10 — µF
Signature Offset Voltage
Tolerance
V OS 0 2.0 V
Signature Offset Current
Tolerance
I OS 0 12 µA