Analog Devices DC981A Demo Manual page 4

DEMO MANUAL
DC981A/DC981B
OPERATING PRINCIPLES
Table 3. Powered Device Combinations
PD COMBINATION
Class 1 / Class 1
Class 1 / Class 2
Class 1 / Class 3**
Class 2 / Class 2
Class 2 / Class 3**
Class 3 **/ Class 3**
**Class 3 substitutable with Class 0 or 4.
If power management is not used, move JP6 to DIS to tie
the PWRMGT pins to VSS and disable this feature.
Class Enforce Mode
ENFORCE CLASS jumper JP1 ties the ENFCLS pin of the
LTC4263 to either VDD5 or VSS to respectively enable or
disable class enforce current limits. If held at VDD5, the
LTC4263 will reduce the ICUT threshold for Class 1 or
Class 2 PDs. If ENFCLS is held at VSS, ICUT remains at
375mA (typical) for all classes.
Table 4. Port Current Limit According to Class
PD CLASS CURRENT THRESHOLD (TYPICAL)
Class 1 100mA
Class 2 175mA
Class 0, 3, 4, or Class 375mA
Enforce Disabled
LED Drive
An LED pin indicates the state of the port controlled by
the LTC4263. When the port is powered, the LED is on;
when disconnected or detecting, the LED is off. If an
invalid signature is detected or a fault occurs, the LED
will flash a pattern that the user or host system can read
to indicate the nature of the problem. When run from a
single 48V supply, the LED pin can operate as a simple
switching current source to reduce power dissipation in
the LED drive circuitry.
4
1ST PD 2ND PD
Powered Powered
Powered Powered
Powered Power Denied
Powered Powered
Powered Power Denied
Powered Power Denied
VDD5 Option
The logic 5V power supply can be supplied from the internal
LTC4263 5V supply or an external 5V supply when above
the internal supply. If the internal regulator is used, this
pin should only be connected to the bypass capacitor and
to logic pins of the LTC4263 held at VDD5.
AC and DC Disconnect
AC and DC disconnect are two different methods of detect-
ing whether a valid PD is present and requires power. AC
disconnect is the default method for the DC981 but can
be converted to DC disconnect in the midspan solution
through two jumpers. Moving DISCON (JP4) to DC will
short the ACCOUT pin to VSS and configure the LTC4263 to
DC disconnect. Moving jumper setting for ACCOMP (JP5)
to DC bypasses the AC blocking diode and removes the
RC used for AC disconnect from the main circuit.
Legacy Detection
LEGACY jumper JP3 controls whether legacy detect is
enabled. If the LEGACY pin is held at VDD5 (EN selected),
legacy detect is enabled and testing for a large capacitor
is performed to detect the presence of a legacy PD on
the port. If held at VSS (DIS selected), only IEEE 802.3af
compliant PDs are detected. If left floating (no jumper),
the LTC4263 enters force-power-on mode and any PD that
generates between 1V and 10V when biased with 270μA of
detection current will be powered as a legacy device. This
mode is useful if the system uses a differential detection
scheme to detect legacy devices. Warning: Legacy modes
are not IEEE 802.3af compliant.
Rev A