Datasheet

LTC4270/LTC4271
26
42701fc
APPLICATIONS INFORMATION
••
3.3V
3.3V 0.1µF
–54V
100Ω
100Ω
100Ω
100Ω
T1
••
3.3V
1µF
1µF
0.1µF
–54V
100Ω
100Ω
100Ω
100Ω
T2
GP0
GP1
MID
RESET
MSD
INT
AUTO
SCL
AD0
AD1
AD2
AD3
AD6
DGND CAP1 CAP2
DND
DPD
CND
CPD
DNA
SENSE1
GATE1
OUT1
LTC4270
DPA
CNA
CPA
XIO0 XIO1
0.22µF
100V
0.22µF
100V
S1B
S1B
–54V
–54V
42701 F16
–54V
–54V
S1B
S1B
PORTn
PORT1
SDAIN
SDAOUT
LTC4271
NO ISOLATION
REQUIRED ON
I
2
C INTERFACE
V
DD33
2nF, 2kV
V
EE
SENSEn
GATEn
OUTn
AGNDVSSK
0.25Ω
0.25Ω
>47µF
SYSTEM
BULK CAP
+
External MOSFET
Careful selection of the power MOSFET is critical to system
reliability. LT C recommends either Fairchild IRFM120A,
FDT3612, FDMC3612 or Philips PHT6NQ10T for their
proven reliability in Type 1 and Type 2 PSE applications.
SOA curves are not a reliable specification for MOSFET
selection. Contact LT C Applications before using a MOSFET
other than one of these recommended parts.
Sense Resistor
The LTC4270/LTC4271 is designed
to use 0.25Ω current
sense resistors to reduce power dissipation. Four com-
monly available 1Ω resistors (sized according to power
dissipation) can be used in parallel in place of a single
Figure 15. LTC4270/LTC4271 Proprietary Isolation
0.25Ω resistor. In order to meet the I
CUT
and I
LIM
accuracy
required by the IEEE specification, the sense resistors
should have ±1% tolerance or better, and no more than
±200ppm/°C temperature coefficient
. In addition, the sense
resistors must meet strict layout guidelines.
Port Output Cap
Each port requires a 0.22μF cap across its outputs to keep
the LTC4270 stable while in current limit during startup
or overload. Common ceramic capacitors often have sig-
nificant voltage coefficients; this means the capacitance is
reduced as the applied voltage increases. To minimize this
problem, X7R ceramic capacitors rated for at least
100V
are recommended and must be located close to the PSE.