Rainbow Electronics MAX5945 - Manual (Page 14)

MAX5945
Setting R19h[PWR_ON_] (Table 21) high immediately
terminates detection/classification routines and turns on
power to the port(s).
R14h[DET_EN_, CLASS_EN_] default to low in SEMI
mode. Use software to set R14h[DET_EN_,
CLASS_EN_] to high to start the detection and/or classi-
fication routines. R14h[DET_EN_, CLASS_EN_] are
reset every time the software commands a power-off of
the port (either through reset or PWR_OFF). In any other
case, the status of the bits is left unchanged (including
when the state machine turns off the power because a
load disconnect or a fault condition is encountered).
MANUAL Mode
Enter MANUAL mode by setting R12h[P_M1,P_M0] to
[0,1] during normal operation (see Tables 15 and 15a).
MANUAL mode allows the software to dictate any
sequence of operation. Write a 1 to both R14h[DET_ EN_]
and R14h[CLASS_EN_] start detection and classifica-
tion operations, respectively, and in that priority order.
After execution, the command is cleared from the regis-
ter(s). PWR_ON_ has highest priority. Setting PWR_ON_
high at any time causes the device to immediately enter
the powered mode. Setting DET_EN and CLASS_EN
high at the same time causes detection to be per-
formed first. Once in the powered state, the device
ignores DET_EN_ or CLASS_EN_ commands.
When switching to MANUAL mode from another mode,
DET_EN_, CLASS_EN_ default to low. These bits
become pushbutton rather than configuration bits (i.e.,
writing ones to these bits while in MANUAL mode com-
mands the device to execute one cycle of detection
and/or classification. The bits are reset back to zeros at
the end of the execution). Putting the MAX5945 into
shutdown mode immediately turns off power and halts
all operations to the corresponding port. The event and
status bits of the affected port(s) are also cleared. In
SHUTDOWN mode, the DET_EN_, CLASS_EN_, and
PWR_ON_ commands are ignored.
In SHUTDOWN mode, the serial interface operates
normally.
Watchdog
R1Dh, R1Eh, and R1Fh registers control watchdog oper-
ation. The watchdog function, when enabled, allows the
MAX5945 to gracefully take over control or securely shut
down the power to the ports in case of software/firmware
crashes. Contact the factory for more details.
PD Detection
When PD detection is activated, the MAX5945 probes
the output for a valid PD. After each detection cycle,
the device sets the DET_END_ bit R04h/05h[3:0] high
and reports the detection results in the status registers
R0Ch[2:0], R0Dh[2:0], R0Eh[2:0], and R0Fh[2:0]. The
DET_END_ bit is reset to low when read through R05h
or after a port reset. Both DET_END_ bit status registers
are cleared after the port powers down.
A valid PD has a 25k Ω discovery signature characteris-
tic as specified in the IEEE 802.3af standard. Table 1
shows the IEEE 802.3af specification for a PSE detect-
ing a valid PD signature (see the Typical Application
Circuit and Figure 2). The MAX5945 can probe and cat-
egorize different types of devices connected 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 MAX5945 turns off the external
MOSFET and forces two probe voltages through the
DET_ input. The current through the DET_ input is mea-
sured 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 connected to the port. The
MAX5945 implements appropriate settling times and a
100ms digital integration to reject 50Hz/60Hz power-
line noise coupling.
An external diode, in series with the DET_ input,
restricts PD detection to the 1st quadrant as specified
by the IEEE 802.3af standard. To prevent damage to
non-PD devices and to protect itself from an output
short circuit, the MAX5945 limits the current into DET_
to less than 2mA maximum during PD detection.
In midspan mode, the MAX5945 waits 2.2s before
attempting another detection cycle after every failed
detection. The first detection, however, happens imme-
diately after issuing the detection command.
Power Device Classification
(PD Classification)
During the PD classification mode, the MAX5945 forces
a probe voltage (-18V) at DET_ and measures the cur-
rent into DET_. The measured current determines the
class of the PD.
After each classification cycle, the device sets the
CL_END_ bit (R04h/05h[7:4]) high and reports the clas-
sification results in the status registers R0Ch[6:4],
R0Dh[6:4], R0Eh[6:4], and R0Fh[6:4]. The CL_END_ bit
is reset to low when read through register R05h or after
a port reset. Both Class_END_ bit status registers are
cleared after the port powers down.
Quad Network Power Controller
for Power-Over-LAN
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