LTC4259CGW Linear Technology, LTC4259CGW Datasheet - Page 15

• Manufacturer Part Number: LTC4259CGW
• Manufacturer: Linear Technology

Specifications of LTC4259CGW

• Linear Misc Type: Negative Voltage
• Operating Supply Voltage (typ): -48V
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 36
• Lead Free Status / Rohs Status: Not Compliant

APPLICATIO S I FOR ATIO
Table 2. IEEE 802.3af Powered Device Classes
17k and 29k (between 19k and 26.5k guaranteed) as a
valid PD and report Detect Good (100 binary) in the Detect
Status bits (bits 2 through 0) of the corresponding Port
Status register. Values outside this range, including open
and short circuits, are also reported in the Detect Status
bits. Refer to Table 1 for a complete decoding of the Detect
Status bits.
The first test point is taken by forcing a test current into
the port, waiting a short time to allow the line to settle and
measuring the resulting voltage. This result is stored and
the second current is applied to the port, allowed to settle
and the voltage measured. Each point takes 100ms to
measure, and an entire detection cycle takes 200ms.
The port’s operating mode controls if and when the
LTC4259 runs a detection cycle. In manual mode, the port
will sit idle until a Restart Detection (register 18h) com-
mand is received. It will then run a complete 200ms
detection cycle on the selected port, report the results in
the Detect Status bits in the corresponding Port Status
register and return to idle until another command is
received. In Semiauto mode, the LTC4259 autonomously
IEEE 802.3fa
CLASS
0
1
2
3
4
275
165
OFFSET
0V-2V
VALID PD
CLASSIFICATION
Figure 13. PD Detection
17mA to 20mA
26mA to 30mA
36mA to 44mA
9mA to 12mA
U
0mA to 4mA
CURRENT
VOLTAGE
25k SLOPE
U
DETECTION
SECOND
POINT
W
PD POWER
DETECTION
MAXIMUM
12.95W
12.95W
12.95W
POINT
3.84W
6.49W
FIRST
4259 F13
U
CLASS DESCRIPTION
Unknown PD Power, PD Does Not Implement Classification
Low Power PD
Medium Power PD
High or Full Power PD
Reserved, Power as Class O
tests valid PDs connected to the ports but it will not apply
power until instructed to do so by the host controller. It
repeatedly queries the port every 320ms and updates the
Detect Status bits at the end of each cycle. If a Detect Good
is reported, it will advance to the classification phase and
report that result in the Port Status register. Until in-
structed to do otherwise, the LTC4259 will continue to
repeat detection on the port. Behavior in Auto mode is
similar to Semiauto; however, after a Detect Good is
reported, the LTC4259 performs the classification phase
and then powers up the port without further intervention.
The signature detection circuitry is disabled when the port
is in Shutdown mode, powered up or the corresponding
Detect Enable bit is cleared.
CLASSIFICATION
A PD has the option of presenting a “classification signa-
ture” to the PSE to indicate how much power it will draw
when powered up. This signature consists of a specific
constant current draw when the PSE port voltage is between
15.5V and 20.5V, with the current level indicating the power
class to which the PD belongs. Per the IEEE 802.3af speci-
fication, the LTC4259 identifies the five classes of PD listed
in Table 2. Note that class 4 is presently specified by the IEEE
as reserved for future use but it is intended to be used with
PDs that digitally communicate additional requirements
over the Ethernet interface after being powered. Figure 14
shows a PD load line, starting with the shallow slope of the
25k signature resistor below 10V then drawing the proper
current for class 3 between 15V and 20V. The LTC4259’s
load line for classification is also shown in Figure 14. It has
low impedance until current limit at 65mA (typ).
LTC4259
15
4259i