Rainbow Electronics MAX5953D - Manual (Page 15)

MAX5953A/MAX5953B/MAX5953C/MAX5953D
IEEE 802.3af PD Interface and PWM Controllers
with Integrated Power MOSFETs
______________________________________________________________________________________ 15
Detailed Description
PD Interface
The MAX5953A/MAX5953B/MAX5953C/MAX5953D
include complete interface function for a PD to comply
with the IEEE 802.3af standard in a PoE system. They
provide the PD with a detection signature, a classifica-
tion signature, and an integrated isolation switch with
programmable inrush current control. An integrated
MOSFET provides PD isolation during detection and
classification. All devices guarantee a leakage current
offset of less than 10µA during the detection phase. A
programmable current limit prevents high inrush cur-
rent during power-on. The device features power-mode
UVLO with wide hysteresis and long deglitch time to
compensate for twisted-pair-cable resistive drop and to
assure glitch-free transition between detection, classifi-
cation, and power-on/-off phases. The MAX5953A/
MAX5953C have an adjustable UVLO threshold with
the default value compliant to the 802.3af standard,
while the MAX5953B/MAX5953D have a lower and
fixed UVLO threshold compatible with some legacy
pre-802.3af PSE.
Operating Modes
Depending on the input voltage (V
IN
= V
+
- V
EE
), the PD
front-end section of the MAX5953A/MAX5953B/
MAX5953C/MAX5953D operate in three different modes:
PD detection signature, PD classification, and PD power.
All voltage thresholds are designed to operate with or
without the optional diode bridge while still complying
with the IEEE 802.3af standard (see Figure 2).
Detection Mode (1.4V ≤ V
IN
≤ 10.1V)
In detection mode, the power source equipment (PSE)
applies two voltages on V
IN in the range of 1.4V to 10.1V
(1V step minimum), and records the corresponding cur-
rent measurements at those two points. The PSE then
computes
∆V/∆I to ensure the presence of the 25.5kΩ
signature resistor. In this mode, most interface circuitry
of the MAX5953A/MAX5953B/MAX5953C/MAX5953D is
off and the offset current is less than 10µA.
Classification Mode (12.6V
≤ V
IN
≤ 20V)
In the classification mode, the PSE classifies the PD
based on the power consumption required by the PD.
This allows the PSE to efficiently manage power distrib-
ution. The IEEE 802.3af standard defines five different
classes as shown in Table 1. An external resistor
(R
RCLASS ) connected from RCLASS to V EE sets the
classification current.
The PSE determines the class of a PD by applying a
voltage at the PD input and measuring the current
sourced out of the PSE. When the PSE applies a volt-
age between 12.6V and 20V, the IC exhibits a current
characteristic with values indicated in Table 2. The PSE
uses the classification current information to classify
the power requirement of the PD. The classification cur-
rent includes the current drawn by the 25.5k
Ω detec-
tion signature resistor and the supply current of the IC
so the total current drawn by the PD is within the IEEE
802.3af standard figures. The classification current is
turned off whenever the device is in power mode.
Table 1. PD Power Classification/
R
RCLASS Selection
Table 2. Setting Classification Current
*Class 4 reserved for future use.
*V IN is measured across the MAX5953A/MAX5953B/MAX5953C/MAX5953D input pins (V+ - V EE ), which do not include the diode
bridge voltage drop.
CLASS USAGE
R RCLASS
( Ω)
MAXIMUM POWER
USED BY PD (W)
0 Default 10k 0.44 to 12.95
1 Optional 732 0.44 to 3.84
2 Optional 392 3.84 to 6.49
3 Optional 255 6.49 to 12.95
4
Not
Allowed
178 Reserved*
CLASS CURRENT SEEN AT V IN (mA)
IEEE 802.3af PD CLASSIFICATION
CURRENT SPECIFICATION (mA) CLASS
R RCLASS
( Ω)
V IN * (V)
MIN MAX MIN MAX
0 10k 12.6 to 20 0 2.00 0 4
1 732 12.6 to 20 9.17 11.83 9 12
2 392 12.6 to 20 17.29 19.71 17 20
3 255 12.6 to 20 26.45 29.55 26 30
4 178 12.6 to 20 36.60 41.40 36 44