ltc4357cms8-trpbf Linear Technology Corporation, ltc4357cms8-trpbf Datasheet - Page 7

• Manufacturer Part Number: ltc4357cms8-trpbf
• Description: Positive High Voltage Ideal Diode Controller
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
MOSFET Selection
The LTC4357 drives an N-channel MOSFET to conduct
the load current. The important features of the MOSFET
are on-resistance, R
voltage, V
Gate drive is compatible with 4.5V logic-level MOSFETs in
low voltage applications (V
ages (V
may be used. An internal clamp limits the gate drive to 15V
between the GATE and IN pins. An external zener clamp
may be added between GATE and IN for MOSFETs with a
V
The maximum allowable drain-source voltage, BV
must be higher than the power supply voltage. If an input
is connected to GND, the full supply voltage will appear
across the MOSFET.
ORing Two Supply Outputs
Where LTC4357s are used to combine the outputs of two
power supplies, the supply with the highest output voltage
sources most or all of the load current. If this supply’s
output is quickly shorted to ground while delivering load
current, the fl ow of current temporarily reverses and
fl ows backwards through the LTC4357’s MOSFET. When
the reverse current produces a voltage drop across the
MOSFET of more than –25mV, the LTC4357’s fast pull-down
activates and quickly turns off the MOSFET.
If the other, initially lower, supply was not delivering load
current at the time of the fault, the output falls until the
body diode of its ORing MOSFET conducts. Meanwhile,
the LTC4357 charges its MOSFET gate with 20μA until the
forward drop is reduced to 25mV. If instead this supply was
delivering load current at the time of the fault, its associ-
ated ORing MOSFET was already driven at least partially
on, and the LTC4357 will simply drive the MOSFET gate
harder in an effort to maintain a drop of 25mV.
When the capacitances at the input and output are very
small, rapid changes in current can cause transients that
exceed the 100V Absolute Maximum Rating of the IN, OUT,
and V
nected from OUT to ground clamps the output and prevents
GS(MAX)
DD
DD
pins. A surge suppressor (TransZorb or TVS) con-
of less than 15V.
DSS
= 20V to 80V) standard 10V threshold MOSFETs
, and the gate threshold voltage.
DS(ON)
DD
, the maximum drain-source
= 9V to 20V). At higher volt-
DSS
,
damage by limiting the magnitude of the peak voltage. In
the absence of a surge suppressor, an output capacitance
of 10μF is suffi cient in most applications to prevent the
transient from exceeding 100V. In lower voltage applica-
tions, the MOSFET’s drain-source breakdown voltage may
be suffi cient to protect the LTC4357 provided BV
< 100V, making a surge suppressor unnecessary.
Load Sharing
The application in Figure 1 combines the outputs of multiple,
redundant supplies using a simple technique known as
droop sharing. Load current is fi rst taken from the highest
output, with the low outputs contributing as the output
voltage falls under increased loading. The 25mV regulation
technique ensures smooth load sharing between outputs
without oscillation. The degree of sharing is a function of
R
initial output voltages.
DS(ON)
PS1
PS2
PS3
RTNA
RTNA
RTNA
V
V
V
48V
48V
48V
INA
INB
INC
, the output impedance of the supplies and their
Figure 1. Droop Sharing Redundant Supplies
IN
IN
IN
FDB3632
FDB3632
FDB3632
LTC4357
LTC4357
LTC4357
GND
GND
GND
GATE
GATE
GATE
OUT
OUT
OUT
V
V
V
DD
DD
DD
LTC4357
4357 F01
DSS
48V BUS
+ V
4357fb
7
IN