LTC4355 LINER [Linear Technology], LTC4355 Datasheet - Page 8

• Manufacturer Part Number: LTC4355
• Description: Positive High Voltage Ideal Diode-OR with Input Supply and Fuse Monitors
• Manufacturer: LINER [Linear Technology]

applicaTions inForMaTion
LTC4355
MOSFET Selection
The LTC4355 drives N-channel MOSFETs to conduct the
load current. The important features of the MOSFETs are
on-resistance R
V
The gate drive for the MOSFET is guaranteed to be greater
than 4.5V when the supply voltage at V
and 20V. When the supply voltage at V
20V, the gate drive is guaranteed to be greater than 10V. The
gate drive is limited to less than 18V. This allows the use
of logic level threshold N-channel MOSFETs and standard
N-channel MOSFETs above 20V. An external zener diode
can be used to clamp the potential from the MOSFET’s
gate to source if the rated breakdown voltage is less than
18V. See the circuit in Figure 4 for an example.
The maximum allowable drain-source voltage, BV
must be higher than the supply voltages. If an input is
connected to GND, the full supply voltage will appear
across the MOSFET.
If the voltage drop across either MOSFET exceeds
the configurable DV
FLT pin and the PWRFLT pin corresponding to the
faulting channel pull low. The R
small enough to conduct the maximum load current
while not triggering a fault, and to stay within the
MOSFET’s power rating at the maximum load current
(I
Fault Conditions
The LTC4355 monitors fault conditions and shunts current
away from LEDs or optocouplers, turning each one off to
indicate a specific fault condition (see Table 1).
When the voltage drop across the pass transistor is higher
than the configurable DV
pulldown at the VDSFLT pin and the PWRFLT1 or PWRFLT2
pin corresponding to the faulting channel turns on. The
DV
SET to GND, tying SET to a 100kW resistor connected
to GND, or floating SET configures DV
500mV, or 1.5V respectively.
Fault conditions that may cause a high voltage across the
pass transistor include: a MOSFET open on the higher

DSS
2
SD(FLT)
• R
, and the threshold voltage.
DS(ON)
threshold is configured by the SET pin. Tying
).
DS(ON)
SD(FLT)
, the maximum drain-source voltage
SD(FLT)
fault threshold, the VDS-
fault threshold, the internal
DS(ON)
OUT
OUT
SD(FLT)
is greater than
is between 9V
should be
to 250mV,
DSS
,
supply, excessive MOSFET current due to overcurrent
on the load or a shorted MOSFET on the lower supply.
During startup or when a switchover between supplies
occurs, the VDSFLT pin and PWRFLT1 or PWRFLT2 pin
may momentarily indicate that the forward voltage has
exceeded the programmed threshold during the short
interval when the MOSFET gate ramps up and the body
diode conducts.
The PWRFLT pins are additionally used to indicate if either
input supply is below its normal regulation range. If the
voltage at the MON1 or MON2 pin is less than V
typically 1.23V, the corresponding PWRFLT1 or PWRFLT2
pin will pull low. A resistive divider connected to the input
supply drives the MON pin for the corresponding supply,
configuring the PWRFLT threshold for that supply. Be sure
to account for the tolerance of the MON pin threshold, the
resistor tolerances, and the regulation range of the supply
being monitored. Also, ensure that the voltage on the MON
pin will not exceed 7V.
The FUSEFLT pins are used to indicate the status of the
input fuses. If one of the IN pins falls below V
cally 3.5V, the FUSEFLT pin corresponding to that supply
will pull low. The IN pins each sink a minimum of 0.5mA,
Table 1. Fault Table
enough to pull the pin low after an input fuse blows open.
If there is a possibility that the MOSFET leakage current
can be greater than 0.5mA, a resistor can be connected
between the IN pin and GND to sink more current. Note
that if the input supply voltage is less than V
FUSEFLT pin will pull low.
*DV
< DV
DV
False
False
False
False
True
True
True
True
SD2
SD(FLT)
SD1
< DV
SD(FLT)
> 3.5V
False
False
False
False
V
True
True
True
True
IN1
> 1.23V
V
False
False
False
False
True
True
True
True
MON1
VDSFLT*
Pulldown
Pulldown
Pulldown
Pulldown
Hi-Z
Hi-Z
Hi-Z
Hi-Z
FUSEFLT1 PWRFLT1
Pulldown
Pulldown
Pulldown
Pulldown
Hi-Z
Hi-Z
Hi-Z
Hi-Z
INx(TH)
INx(TH)
MON(TH)
Pulldown
Pulldown
Pulldown
Pulldown
Pulldown
Pulldown
Hi-Z
Hi-Z
, typi-
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