LTC4267CDHC Linear Technology, LTC4267CDHC Datasheet - Page 12

• Manufacturer Part Number: LTC4267CDHC
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,CMOS,SSOP,16PIN,PLASTIC
• Manufacturer: Linear Technology

Specifications of LTC4267CDHC

• Linear Misc Type: Negative Voltage
• Package Type: DFN EP
• Operating Supply Voltage (max): -57V
• Operating Temperature (min): 0C
• Operating Temperature (max): 70C
• Operating Temperature Classification: Commercial
• Product Depth (mm): 3mm
• Product Length (mm): 5mm
• Mounting: Surface Mount
• Pin Count: 16
• Lead Free Status / Rohs Status: Not Compliant

LTC4267
APPLICATIO S I FOR ATIO
Classifi cation
Once the PSE has detected a PD, the PSE may option-
ally classify the PD. Classifi cation provides a method for
more effi cient allocation of power by allowing the PSE
to identify lower power PDs and allocate less power for
these devices. The IEEE 802.3af specifi cation defi nes fi ve
classes (Table 2) with varying power levels. The designer
selects the appropriate classifi cation based on the power
consumption of the PD. For each class, there is an as-
sociated load current that the PD asserts onto the line
during classifi cation probing. The PSE measures the PD
load current to determine the proper classifi cation and
PD power requirements.
During classifi cation (Figure 4), the PSE presents a fi xed
voltage between –15.5V and –20.5V to the PD. With the
input voltage in this range, the LTC4267 asserts a load
current from the V
The magnitude of the load current is set by the R
resistor. The resistor values associated with each class
are shown in Table 2. Note that the switching regulator
will not interfere with the classifi cation measurement since
the LTC4267 has not passed power to the regulator.
Table 2. Summary of IEEE 802.3af Power Classifi cations and
LTC4267 R
Class
*Class 4 is currently reserved and should not be used.
12
0
1
2
3
4
PSE
TO
Reserved
Optional
Optional
Optional
Default
Usage
Figure 3. 25k Signature Resistor with Disable
CLASS
V
LTC4267
25k SIGNATURE
PORTN
Resistor Selection
RESISTOR
at Input of PD
Power Levels
0.44 to 12.95
6.49 to 12.95
0.44 to 3.84
3.84 to 6.49
Maximum
Reserved*
PORTP
U
(W)
pin through the R
9k
16k
U
SIGNATURE DISABLE
Classifi cation
Load Current
Nominal
(mA)
10.5
18.5
W
<5
28
40
SIGDISA
V
PORTP
CLASS
4267 F03
U
LTC4267
(Ω, 1%)
Resistor
R
Open
resistor.
68.1
45.3
30.9
CLASS
124
CLASS
The IEEE 802.3af specifi cation limits the classifi cation
time to 75ms because a signifi cant amount of power is
dissipated in the PD. The LTC4267 is designed to handle the
power dissipation for this time period. If the PSE probing
exceeds 75ms, the LTC4267 may overheat. In this situation,
the thermal protection circuit will engage and disable the
classifi cation current source in order to protect the part.
The LTC4267 stays in classifi cation mode until the input
voltage rises above the UVLO turn-on voltage.
V
The IEEE specifi cation dictates a maximum turn-on voltage
of 42V and a minimum turn-off voltage of 30V for the PD.
In addition, the PD must maintain large on-off hysteresis
to prevent resistive losses in the wiring between the PSE
and the PD from causing start-up oscillation. The LTC4267
incorporates an undervoltage lockout (UVLO) circuit that
monitors the line voltage at V
to apply power to the integrated switching regulator
(Figure 5). Before the power is applied to the switching
regulator, the P
the ground potential since there is no charge on capacitor
C1. When the input voltage rises above the UVLO turn-on
threshold, the LTC4267 removes the detection and clas-
sifi cation loads and turns on the internal power MOSFET.
C1 charges up under the LTC4267 current limit control
and the P
sequence is shown in Figure 1. The LTC4267 includes
a hysteretic UVLO circuit on V
applied to the load until the input voltage falls below the
UVLO turn-off threshold. Once the input voltage drops
below –30V, the internal power MOSFET is turned off and
–15.5V TO –20.5V
PORTN
PROBING
VOLTAGE
SOURCE
Undervoltage Lockout
Figure 4. IEEE 802.3af Classifi cation Probing
PSE
OUT
PSE CURRENT MONITOR
pin transitions from 0V to V
OUT
PSE
V
pin is high impedance and sitting at
CURRENT PATH
R
CLASS
PORTN
PORTN
R
V
LTC4267
CLASS
PORTN
PD
to determine when
that keeps power
V
PORTP
4267 F04
PORTN
CONSTANT
LOAD
CURRENT
INTERNAL
TO LTC4267
. This
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