Datasheet

LTC4362-1/LTC4362-2
10
436212fa
APPLICATIONS INFORMATION
Figure 6. Setup for Testing 20V Plugged into 5V System
Figure 7. Overvoltage Protection Waveforms
When 20V Plugged into 5V System
Figure 8. Overvoltage Protection Waveforms When
20V Plugged into 5V System with External IN Clamp
Figure 5 shows an input transient after an overcurrent.
The current in L
IN
will cause V
IN
to overshoot and avalanche
the internal N-channel MOSFET to C
OUT
.
adaptor is mistakenly hot-plugged into the 5V device with
the USB input already live. As shown in Figure 7, a large
current can build up in L
IN
to charge up C
OUT
. When the
internal MOSFET shuts off, this current is dumped into
C
OUT
, causing a large 40V transient. The LTC4362 limits
this to a 1V rise in the output voltage.
Figure 5. Input Transient After Overcurrent
2µs/DIV
V
IN
10V/DIV
V
OUT
5V/DIV
I
CABLE
2A/DIV
FIGURE 4 CIRCUIT
R
IN
= 150m, L
IN
= 0.7µH
LOAD = 10, C
OUT
= 10µF
436212 F05
TURN-OFF
OVERCURRENT
C
OUT
436212 F06
20V
WALL
ADAPTOR
5V
USB
LT4362
GND
LOAD
OUTIN
OUTIN
B160
R
IN
I
CABLE
L
IN
+
–
+
–
1µs/DIV
V
IN
10V/DIV
V
OUT
2V/DIV
I
CABLE
10A/DIV
R
IN
= 150m, L
IN
= 2µH
LOAD = 10
C
OUT
= 10µF (16V, SIZE 1210)
436212 F07
1µs/DIV
V
IN
10V/DIV
V
OUT
2V/DIV
I
CABLE
10A/DIV
R
IN
= 150m, L
IN
= 0.7µH
LOAD = 10
C
OUT
= 10µF (16V, SIZE 1210)
436212 F08
If the voltage rise at V
OUT
due to the discharge of the energy
in L
IN
into C
OUT
is not acceptable or the avalanche capability
of the MOSFET is exceeded, an additional external clamp
Z1 such as the SMAJ24A can be placed between IN and
GND. Figure 8 shows the resulting waveform.
Typically, IN will be clamped to a voltage of V
OUT
+ 1.3•(30V
BV
DSS
of Internal MOSFET) = 45V. The single, nonrepeti-
tive, pulse of energy (E
AS
) absorbed by the MOSFET dur-
ing this avalanche breakdown with a peak current I
AS
is
approximated by the formula:
E
AS
=
1
2
•L
IN
•I
AS
2
For L
IN
= 0.7µH and I
AS
= 3A, then E
AS
= 3.15µJ. This is
within the I
AS
and E
AS
capabilities of the internal MOSFET.
So in most instances, the LTC4362 can ride through such
transients without a bypass capacitor, transient voltage
suppressor or other external components at IN.
Figure 6 shows a particularly bad situation which can oc-
cur in a mobile device with dual power inputs. A 20V wall