LTC4266CUHF-PBF View Datasheet(PDF) - Linear Technology
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LTC4266CUHF-PBF Datasheet PDF : 30 Pages
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LTC4266
APPLICATIONS INFORMATION
at 1/16 the rate that it counts up. This allows the current
limit circuitry to tolerate intermittent overload signals with
duty cycles below about 6%; longer duty cycle overloads
will turn the port off.
ICUT is typically set to a lower value than ILIM to allow the
port to tolerate minor faults without current limiting.
Per the IEEE specification, the LTC4266 will automatically
set ILIM to 425mA (shown in bold in Table 5) during in-
rush at port turn-on, and then switch to the programmed
ILIM setting once inrush has completed. To maintain IEEE
compliance, ILIM should kept at 425mA for all Type 1 PDs,
and 850mA if a Type 2 PD is detected. ILIM is automatically
reset to 425mA when a port turns off.
Table 5. Example Current Limit Settings
ILIM (mA)
53
INTERNAL REGISTER SETTING (hex)
RSENSE = 0.5Ω
88
RSENSE = 0.25Ω
106
08
88
159
89
213
80
08
266
8A
319
09
89
372
8B
425
00
80
478
8E
531
92
8A
584
CB
638
10
90
744
D2
9A
850
40
C0
956
4A
CA
1063
50
D0
1169
5A
DA
1275
60
E0
1488
52
49
1700
40
1913
4A
2125
50
2338
5A
2550
60
2975
52
ILIM Foldback
The LTC4266 features a two-stage foldback circuit that
reduces the port current if the port voltage falls below the
normal operating voltage. This keeps MOSFET power dis-
sipation at safe levels for typical 802.3af MOSFETs, even at
extended 802.3at power levels. Current limit and foldback
behavior are programmable on a per-port basis. Figure
14 shows MOSFET power dissipation with 802.3af-style
foldback compared with a typical MOSFET SOA curve;
Figure 15 demonstrates how two-stage foldback keeps
the FET within its SOA under the same conditions. Table 5
gives examples of recommended ILIM register settings.
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
802.3af FOLDBACK
SOA DC AT 90°C
10 20 30 40 50 60
PD VOLTAGE (V) AT VPSE = 58V
4266 F14
Figure 14. Turn On Currents vs FET Safe Operating
Area at 90°C Ambient
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0
SOA 75ms AT 90°C
802.3af FOLDBACK
SOA DC AT 90°C
10 20 30 40 50 60
PD VOLTAGE (V) AT VPSE = 58V
4266 F15
Figure 15. LTC4266 Foldback vs FET Safe Operating
Area at 90°C Ambient
4266fb
20
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