Datasheet

LTC4360-1/LTC4360-2
10
436012fa
Figure 8. Set-Up for Testing 20V Plugged into 5V System
LOAD
436012 F07
OUT
C
OUT
M1
Si1470DH
IN
L
IN
R
IN
20V
WALL
ADAPTER
5V
USB
LTC4360
IN
R1
100k
D1
B160
OUT
GND
GATE
+
–
+
–
I
CABLE
APPLICATIONS INFORMATION
Figure 7. Input Transient After Overvoltage
V
ADAPTOR
/V
OUT
5V/DIV
V
IN
20V/DIV
I
CABLE
5A/DIV
2µs/DIV
436012 F06
FIGURE 5 CIRCUIT
R
IN
= 150µΩ, L
IN
= 2µH
LOAD = 10Ω, C
OUT
= 10µF
V
ADAPTOR
V
OUT
As the IN pin can withstand up to 80V, a high voltage N-
channel MOSFET can be used to protect the system against
rugged abuse from high transient or DC voltages up to
the BV
DSS
of the MOSFET. Figure 6 shows a 50V input
plugged into the LTC4360 controlling a 60V rated MOSFET.
Input transients also occur when the current through the
cable inductance changes abruptly. This can happen when
the LTC4360 turns off the N-channel MOSFET rapidly in an
overvoltage event. Figure 7 shows the effects of a voltage
transient at the wall adaptor output V
ADAPTOR
. The current in
L
IN
will cause V
IN
to overshoot and avalanche the N-channel
MOSFET to C
OUT
. Typically, IN will be clamped to a voltage
of V
OUT
+ 1.3•(BV
DSS
of Si1470DH) = 45V. This is well
below the 85V absolute maximum voltage rating of the
LTC4360. The single, nonrepetitive, pulse of energy (E
AS
)
absorbed by the MOSFET during this avalanche breakdown
with a peak current I
AS
is approximated by the formula:
E
AS
= 0.5 • L
IN
• I
AS
2
For L
IN
= 2µH and I
AS
= 4A, then E
AS
= 16µJ. This is within
the I
AS
and E
AS
capabilities of most MOSFET’s including
the Si1470DH. So in most instances, the LTC4360 can
ride through such transients without a bypass capacitor,
transient voltage suppressor or other external components
at IN.
Figure 8 shows a particularly severe situation which can
occur in a mobile device with dual power inputs. A 20V
wall adaptor is mistakenly hot-plugged into the 5V device
Figure 6. 50V Hot-Plug into the LTC4360
V
IN
20V/DIV
V
OUT
1V/DIV
I
CABLE
5A/DIV
5µs/DIV
436012 F05
FDC5612
R
IN
= 150mΩ, L
IN
= 0.7µH
LOAD = 10Ω, C
OUT
= 10µF