Datasheet

LTC4266
20
4266fe
For more information www.linear.com/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.
I
CUT
is typically set to a lower value than I
LIM
to allow the
port to tolerate minor faults without current limiting.
Per the IEEE specification, the LTC4266 will automatically
set I
LIM
to 425mA (shown in bold in Table 5) during in-
rush at port turn-on, and then switch to the programmed
I
LIM
setting once inrush has completed. To maintain IEEE
compliance, I
LIM
should kept at 425mA for all Type 1 PDs,
and 850mA if a Type 2 PD is detected. I
LIM
is automatically
reset to 425mA when a port turns off.
Table 5. Example Current Limit Settings
I
LIM
(mA)
INTERNAL REGISTER SETTING (hex)
R
SENSE
= 0.5Ω R
SENSE
= 0.25Ω
53 88
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
I
LIM
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 I
LIM
register settings.
PD VOLTAGE (V) AT V
PSE
= 58V
0
0.0
PSE CURRENT (A)
0.2
0.4
0.6
10
20
30 40
4266 F14
50
0.8
1.0
0.1
0.3
0.5
0.7
0.9
60
802.3af FOLDBACK
2 x 802.3af FOLDBACK
SOA 75ms AT 25°C
SOA DC AT 90°C
PD VOLTAGE (V) AT V
PSE
= 58V
0
0.0
PSE CURRENT (A)
0.2
0.4
0.6
10
20
30 40
4266 F15
50
0.8
1.0
0.1
0.3
0.5
0.7
0.9
60
LTC4266 FOLDBACK
802.3af FOLDBACK
SOA DC AT 90°C
SOA 75ms AT 90°C
SOA 75ms AT 25°C
Figure 14. Turn On Currents vs FET Safe Operating
Area at 90°C Ambient
Figure 15. LTC4266 Foldback vs FET Safe Operating
Area at 90°C Ambient