LM5070MTCX-50 National Semiconductor, LM5070MTCX-50 Datasheet - Page 11

• Manufacturer Part Number: LM5070MTCX-50
• Description: Integrated Power Over Ethernet PD Interface and PWM Controller
• Manufacturer: National Semiconductor

An external signature resistor is connected to V
exceeds 1.8V, initiating detection mode. During detection
mode, quiescent current drawn by the LM5070 is less than
10uA. Between 10.0V and 12.5V, the device enters classifi-
cation mode and the signature resistor is disabled. The
nominal range for classification mode is 11.5V to 21.5V. The
classification current is turned off once the classification
range voltage is exceeded, to reduce power dissipation.
Between 21.5V and UVLO release, the device is in a
standby state, awaiting the input voltage to reach the opera-
tional range to complete the power up sequence. Once the
V
age, the internal power MOSFET is enabled to deliver a
constant current to charge the input capacitor of the dc-dc
converter. When the MOSFET Vds voltage falls below 1.5V,
the internal Power Good signal enables the SMPS controller.
The LM5070 is specified to operate with an input voltage as
high as 75V. The SMPS controller and internal MOSFET are
disabled when V
Detection Signature
To detect a potential powered device candidate, the PSE will
apply a voltage from 2.8V to 10V across the input terminals
of the PD. The voltage can be of either polarity so a diode
barrel network is required on both lines to ensure this capa-
bility. The PSE will take two measurements, separated by at
least 1V and 2ms of time. The voltage ramp between mea-
surement points will not exceed 0.1V/us. The delta voltage /
delta current calculation is then performed; if the detected
impedance is above 23.75kΩ and below 26.25kΩ, the PSE
will consider a PD to be present. If the impedance is less
than 15kΩ or greater than 33kΩ a PD will be considered not
present and will not receive power. Impedances between
these values may or may not indicate the presence of a valid
PD. The LM5070 will enable the signature resistor at a
controller input voltage of 1.5V to take into account the diode
voltage drops. The PSE will tolerate no more than 1.9V of
offset voltage (caused by the external diodes) or more than
10uA of offset current (bias current). The input capacitance
must be greater than 0.05uF and less than 0.12uF. To in-
crease efficiency, the signature resistor is disabled by the
LM5070 controller once the input voltage is above the de-
tection range (
Classification
To classify the PD, the PSE will present a voltage between
14.5V and 20.5V to the PD. The LM5070 enables classifica-
tion mode at a nominal input voltage of 11.5V. An internal
1.5V linear regulator and an external resistor connected to
the RCLASS pin provide classification programming current.
Table 2 shows the external classification resistor required for
a particular class.
IN
TABLE 1. Operating Modes With Respect to Input
voltage increases above the upper UVLO threshold volt-
12.5V to 20.5V
23.0V to UVLO
Input Voltage
1.8V to 10.0V
75V to UVLO
V
>
Falling Vth
Rising Vth
IN
IN
11V).
wrt V
falls to the lower UVLO threshold.
EE
Voltage
Normal Powered
Classification
Awaiting Full
(Signature)
Operation
Operation
Detection
Mode of
Power
EE
when V
IN
11
The classification current flows through the IC into the clas-
sification resistor. The suggested resistor values take into
account the bias current flowing into the IC. A different
desired RCLASS can be calculated by dividing 1.5V by the
desired classification current.
Per the IEEE 802.3af specification, classification is optional,
and the PSE will default to class 0 if a valid classification
current is not detected. If PD classification is not desired
(i.e., Class 0), simply leave the RCLASS pin open. The
classification time period may not last longer then 75ms as
per IEEE 802.3af. The LM5070 will remain in classification
mode until V
Undervoltage Lockout (UVLO)
The IEEE 802.3af specification states that the PSE will
supply power to the PD within 400ms after completion of
detection. The LM5070 contains a programmable line Under
Voltage Lock Out (UVLO) circuit. The first resistor should be
connected between the V
tor in the divider should be connected between the UVLO
and UVLORTN pins. The bottom resistor should not be tied
to V
system to violate the 10uA maximum offset current specifi-
cation during detection mode.
The divider must be designed such that the voltage at the
UVLO pin equals 2.0V when V
mum operating level. If the UVLO threshold is not met, the
interface control and SMPS control will remain in standby.
UVLO hysteresis is accomplished with an internal 10uA
current source that is switched on and off into the impedance
of the UVLO set point divider. When the UVLO threshold is
exceeded, the current source is activated to instantly raise
the voltage at the UVLO pin. When the UVLO pin voltage
falls below the 2.00V threshold, the current source is turned
off, causing the voltage at the UVLO pin to fall. The LM5070
UVLO thresholds cannot be programmed lower than 23V,
otherwise the device would operate in classification mode
with both the classification current source and the SMPS
enabled. The combined power dissipation of these two func-
tions could exceed the maximum power dissipation of the
package.
There are many additional uses for the UVLO pin. The UVLO
function can also be used to implement a remote enable /
disable function. Pulling the UVLO pin down below the
UVLO threshold disables the interface and SMPS controller.
Power Supply Operation / Current
Limit Programming
Once the UVLO threshold has been satisfied, the interface
controller of the LM5070 will charge up the SMPS input
capacitor through the internal power MOSFET. This load
TABLE 2. Classification Levels and Required External
Class
0
1
2
3
4
EE
because any current from V
Reserved Reserved
0.44W
0.44W
3.84W
6.49W
PMIN
IN
is greater than 22V.
12.95W
12.95W
PMAX
3.84W
6.49W
Resistors
IN
to UVLO pins; the bottom resis-
IN
ICLASS
17mA
26mA
36mA
(MIN)
0mA
9mA
reaches the desired mini-
IN
to V
ICLASS
(MAX)
12mA
20mA
30mA
44mA
EE
4mA
will cause the
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RCLASS
82.5Ω
53.6Ω
38.3Ω
Open
150Ω