MAX5922
+48V, Single-Port Network Power Switch
For Power-Over-LAN
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Detailed Description
The MAX5922 is a single-port network power controller
with an integrated power MOSFET, operating from a
+32V to +60V supply rail. The device is specifically
designed for PSE in power-over-LAN applications and is
fully compliant to the IEEE 802.3af standard. The
MAX5922 provides PD discovery, classification, current
limit, and other necessary functions for an IEEE 802.3af-
compliant PSE.
The MAX5922 operates in three different modes: PD
detection mode, PD classification mode, and power
mode. Figures 2 and 4 illustrates the device’s functional
operation.
PD Detection Mode
Once powered up and enabled, the MAX5922 probes
the output for a valid PD. A valid PD should have a 25kΩ
discovery signature characteristic as specified in the
IEEE 802.3af standard. Table 1 shows the IEEE 802.3af
specification for a PSE detection of PDs (see the Typical
Application Circuit and Figure 3 (MAX5922 startup
sequence)).
The MAX5922 performs the PD detection by forcing a
probe voltage (V
PBI
= 4V) at the OUT pin and senses the
current out of this pin. The sensed current is sampled and
held t
DET
(88ms) after the probing voltage is sent. The
probe voltage is then switched to V
PBII
= 8V. At the end
of another t
DET
period, the ratio of the difference of the
two test voltages and sensed currents (∆V/∆I) is calculat-
ed to determine the PD resistance. The MAX5922 PD
detection circuitry checks for a valid PD resistive signa-
ture between 19kΩ and 26.5kΩ, with a parallel capaci-
tance of up to 0.6µF. The MAX5922 PD detection circuit
rejects all PDs showing resistive signature of less than
15kΩ or greater than 33kΩ, and/or a capacitive signature
greater than 6µF. Any resistive signature between 15kΩ
to 19kΩ, or between 26.5kΩ to 33kΩ, and/or a capaci-
tance between 0.6µF to 6µF can produce unpredictable
detection results. If the MAX5922 does not detect a valid
PD signature, it continually sends the probe voltages to
the output indefinitely (see Figure 5).
The detection current reference is set by an external
resistor (R
RDT)
) connected from the RDT pin to AGND.
This resistor should be an 18.2kΩ ±1%, with an optional
680nF capacitor in parallel for filtering out power-line
noise. An internal diode in series with the detection volt-
age source and OUT is provided to restrict PD detec-
tion to the 1st quadrant as specified by the IEEE
standard 802.3af (see Figure 3). To prevent damage to
non-PD devices and to protect itself from output short
circuit, the MAX5922 limits the current out of the OUT
pin during PD detection to 1.5mA (max).
For midspan systems where power is delivered to the
PD through the spare pairs, the detection collision
avoidance must be activated. In this mode, after every
failed PD detection cycle, the MAX5922A/MAX5922C
enter a back-off mode where they drive the OUT pin
into high impedance for t
DCA
(2.8s). The DCA pin must
be connected high (MAX5922A) to activate the detec-
tion collision avoidance circuitry (if connected low, the
detection collision avoidance circuitry is disabled). The
MAX5922C has the detection collision avoidance cir-
cuitry permanently enabled (see the Typical
Application Circuit). After t
DCA
, the MAX5922A (with
DCA high) and the MAX5922C resume PD detection
operation. The MAX5922B has the detection collision
avoidance circuitry permanently disabled.
Detection Enable/Disable
The MAX5922A has the PD detection mode permanently
enabled. The MAX5922B/MAX5922C are equipped with
a DET_DIS pin, which provides the option of enabling or
disabling the power-device detection phase. With the
DET_DIS pin connected high, the PD detection and clas-
sification phases are disabled regardless of the status of
the class pin. With the DET_DIS pin connected low, the
PD detection is enabled.
Table 1. IEEE802.3af PD Specification
