Rainbow Electronics MAX5971А User Manual
Page 18
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MAX5971A
Single-Port, 40W, IEEE 802.3af/802.3at PSE
Controller with Integrated MOSFET
Digital Logic
The MAX5971A internally generates digital supplies
(referenced to V
EE
) to power the internal logic circuitry.
All logic inputs and outputs are referenced to V
EE
.
See the Electrical Characteristics table for digital input
thresholds. If digital logic inputs are driven externally, the
nominal digital logic level is 3.3V.
Undervoltage and Overvoltage Protection
The MAX5971A contains undervoltage and overvoltage
protection features. An internal V
EE
undervoltage lockout
(V
EE_UVLO
) circuit keeps the port off and the MAX5971A in
reset until V
AGND
- V
EE
exceeds 28.5V (typ) for more than
2.5ms. An internal V
EE
overvoltage (V
EE_OV
) circuit shuts
down the port when (V
AGND
- V
EE
) exceeds 62.5V (typ).
DC Disconnect Monitoring
Force OSC to V
EE
and power or reset the device to acti-
vate DC load-disconnect monitoring. If I
RSENSE
(the cur-
rent across R
SENSE
) falls below the DC load-disconnect
threshold, I
DCTH
, for more than t
DISC
, the device turns
off port power.
AC Disconnect Monitoring
The MAX5971A features AC load-disconnect monitor-
ing. Bypass OSC with a 100nF (Q10% tolerance) exter-
nal capacitor to V
EE
and power or reset the device to
enable AC disconnect. When AC disconnect is enabled,
a blocking diode in series to OUT and an RC circuit in
parallel to the DET diode must be used, as shown in the
typical operating circuit of Figure 7.
The AC disconnect uses an internal triangle wave gen-
erator to supply the probing signal. Then the resulting
4V
P-P
amplitude wave is forced on DET. The common
mode of the output signal probed on DET is 5V below
AGND. If the AC current peak into DET falls below I
ACTH
for more than t
DISC
, the device powers down the port.
PWM and LED Signals
The MAX5971A includes a multifunction LED driver to
inform the user of the port status. LED is an open-drain,
multifunction output referenced to V
EE
and can sink up
to 10mA (typ) while driving an external LED. The LED is
turned on when the port is connected to a valid PD and
powered. If the port is not powered or is disconnected,
the LED will be off.
For two other conditions, the MAX5971A blinks a code to
communicate the port status. A series of two flashes indi-
cates an overcurrent fault occurred during port power-on,
and has a timing characteristic detailed by Figure 4. A
series of five flashes indicates that during detection an
invalid low or high discovery signature resistance was
detected and has a timing characteristic detailed by
Figure 5.
Figure 4. LED Code Timing for Overcurrent Fault During Port Power-On
Figure 5. LED Code Timing for Detection Fault Due to High- or Low-Discovery Signature Resistance
LED OFF
LED ON
223ms
LED ON
LED ON
LED ON
LED OFF
LED OFF
74ms
74ms
223ms
PORT POWERED DOWN, DUE TO OVERCURRENT FAULT
PORT POWERED ON
PORT POWERED ON AGAIN
LED
OFF
LED
ON
223ms
74ms
LED
OFF
LED
ON
223ms
74ms
LED
OFF
LED
ON
223ms
74ms
LED
OFF
LED
ON
223ms
74ms
LED
ON
74ms
LED
OFF
1.4s
SEQUENCE REPEATS
INVALID HIGH OR LOW
DISCOVERY SIGNATURE
RESISTANCE DETECTED