MAX5984 MAXIM [Maxim Integrated Products], MAX5984 Datasheet - Page 14

• Manufacturer Part Number: MAX5984
• Description: Single-Port, 40W, IEEE 802.3af/at PSE Controller with Integrated MOSFET
• Manufacturer: MAXIM [Maxim Integrated Products]

Table 1. PSE PD Detection Modes Electrical Requirements (IEEE 802.3at)
Single-Port, 40W, IEEE 802.3af/802.3at PSE
Controller with Integrated MOSFET
During a reset, the MAX5984 latches in the state of
MIDSPAN, LEGACY, and OSC. During normal operation,
changes to these inputs are ignored.
In midspan mode, the device adopts cadence timing
during the detection phase. When cadence timing is
enabled and a failed detection occurs, the port waits
between 2s and 2.4s before attempting to detect again.
Midspan mode is activated by setting MIDSPAN high
and then powering or resetting the device. By default,
the MIDSPAN input is internally pulled high. Force
MIDSPAN low to disable this function.
The MAX5984 operates automatically after the reset
condition is cleared. The device performs detection and
classification, and powers up the port automatically once
a valid PD is detected at the port. If a valid PD is not con-
nected at the port, the MAX5984 repeats the detection
routine continuously until a valid PD is connected.
During normal operation, the MAX5984 probes the output
for a valid PD. A valid PD has a 25kI discovery signature
characteristic as specified in the IEEE 802.3af/802.3at
standard. Table 1 shows the IEEE 802.3at specification
for a PSE detecting a valid PD signature.
14
Open-circuit voltage
Short-circuit current
Valid test voltage
Voltage difference between test points
Time between any two test points
Slew rate
Accept signature resistance
Reject signature resistance
Open-circuit resistance
Accept signature capacitance
Reject signature capacitance
Signature offset voltage tolerance
Signature offset current tolerance
_____________________________________________________________________________________
PARAMETER
Automatic Operation
Midspan Mode
PD Detection
SYMBOL
R
C
DV
V
R
V
R
C
V
GOOD
GOOD
VALID
V
SLEW
OPEN
I
I
t
BAD
BAD
OS
SC
TEST
BP
OC
OS
< 15
MIN
500
2.8
19
10
1
2
0
0
During detection, the MAX5984 keeps the internal
MOSFET off and forces two probe voltages through DET.
The current through DET is measured as well as the volt-
age at OUT. A two-point slope measurement is used, as
specified by the IEEE 802.3af/802.3at standard, to verify
the device connected to the port.
An external diode, in series with the DET input, restricts
PD detection to the first quadrant as specified by the
IEEE 802.3af/802.3at standard. To prevent damage to
non-PD devices, and to protect itself from an output short
circuit, the MAX5984 limits the current into DET to less
than 2mA maximum during PD detection.
In midspan mode, the MAX5984 waits at least 2.0s
before attempting another detection cycle after every
failed detection. The first detection, however, happens
immediately after exiting a reset condition.
The status of the LEGACY input is latched during power-
up or after reset condition is cleared. The LEGACY
input is internally pulled high enabling high-capacitance
detection. Unless high-capacitance detection is needed,
connect LEGACY to V
capacitance detection is enabled, PD signature capaci-
tances up to 47FF (typ) are accepted.
MAX
> 33
26.5
150
0.1
2.0
30
10
12
5
UNITS
V/Fs
mA
ms
kI
kI
kI
FA
nF
FF
V
V
V
V
EE
to disable this function. If high-
High-Capacitance Detection
In detection mode only
In detection mode only
This timing implies a 500Hz
maximum probing frequency
ADDITIONAL INFORMATION