Datasheet

LTC4366-1/LTC4366-2
436612fd
For more information www.linear.com/LTC4366
13
applicaTions inForMaTion
Following an overvoltage fault, putting the part in shutdown
will clear the fault, allowing operation to resume once the
LTC4366 leaves shutdown.
Output Short
A sudden short on the output can result in excessive cur
-
rent into the LTC4366 GATE pin supplied from the gate
capacitor
, C
G
. The GATE pin is internally clamped to OUT
with a 10V to 12V clamp. If the OUT pin is pulled low
while the GATE pin is held up with C
G
, then the clamp will
be damaged trying to discharge C
G
when clamp voltage
is exceeded. One solution is to add a 1k R
S
resistor in
series with C
G
with a bypass diode as shown in Figure 3.
The diode allows the capacitor to function as a bypass for
energy coming from the MOSFET drain to gate capacitor
during an supply overvoltage.
GATE
D
BYPASS
R
S
1k
C
G
436612 F03
R
G
M1
LTC4366
OUT
10V TO
12V
Figure 3. Output Short Protection
LTC4366
BASE
436612 F04
R
SS
V
SS
Figure 4. External PNP Option
Resistor Power Ratings
The proper rating for the R
SS
resistor in Figure 1 must be
considered. During an overvoltage event the OUT pin is
at regulation voltage (V
REG
), so the voltage across R
SS
is
V
REG
minus 5.7V. A small minimum supply voltage reduces
the value of R
SS
. Therefore, large differences between
minimum supply voltage and the regulation voltage may
require a large power resistor for R
SS
.
The full supply voltage minus 12V can appear across
R
IN
during the overvoltage cool-down period. Normally
the value for R
IN
is several times larger than R
SS
which
lowers the power and size requirements for this resistor.
External PNP
In some cases the power resistor for R
SS
may be physically
large. A large value R
SS
(with lower power and size) may
be used in conjunction with a PNP as shown in Figure4.
In addition to the 0.8µA sourced from the BASE pin, the
base current from the PNP must flow through R
SS
which
will limit the maximum R
SS
value. In some cases the
minimum PNP Beta is as low as 35. The base current
becomes 10µA when the V
SS
current is 350µA. One can
see this allows a 35 (Beta) times larger R
SS
than the ap-
plication without the PNP.
Minimum Supply Start-Up
When designing for the minimum supply condition, it is
important that R
SS
and R
IN
are chosen to provide enough
current to sufficiently charge C1 to 4.75V. The parameters
that determine the minimum supply voltage include: C1
voltage, MOSFET threshold voltage, a series Schottky
diode voltage drop, resistance of R
SS
and R
IN
, current in