LTC1154
10
1154fc
For more information www.linear.com/LTC1154
applicaTions inForMaTion
The Y axis of the graph is normalized to one RC time
constant. The X axis is normalized to the current. (The
set current is defined as the current required to develop
100mV across the drain sense resistor).
Note that the shutdown time is shorter for increasing levels
of MOSFET current. This ensures that the total energy
dissipated by the MOSFET is always within the bounds
established by the manufacturer for safe operation. (See
MOSFET data sheet for further information).
Using a Speed-Up Diode
To reduce the amount of time that the power MOSFET is
in a short-circuit condition, “bypass” the delay resistor
with a small signal diode as shown in Figure 6. The diode
will engage when the drop across the drain sense resistor
exceeds about 0.7V, providing a direct path to the sense pin
and dramatically reducing the amount of time the MOSFET
is in an overload condition. The drain sense resistor value
is selected to limit the maximum DC current to 2.8A. The
diode conducts when the drain current exceeds 20A and
reduces the turn-off time to 15µs.
Reverse Battery Protection
The LTC1154 can be protected against reverse battery
conditions by connecting a resistor in series with the
ground lead as shown in Figure 7. The resistor limits the
supply current to less than 50mA with –12V applied.
Figure 6. Using a Speed-Up Diode
IN
EN
STATUS
GND
V
S
DS
G
SD
LTC1154
+
100µF
IRF530
15V
0.036Ω
LTC1154 • F06
0.01µF
100k
1N4148
LOAD
Since the LTC1154 draws very little current while in normal
operation, the drop across the ground resistor is minimal.
The 5V µP (or control logic) is protected by the 10k resis
-
tors in series with the input and status pins.
Current Limited Power Supplies
The LTC1154 requires at least 3.5V at the supply pin to
ensure proper operation. It is therefore necessary that the
supply to the LTC1154 be held higher than 3.5V at all times,
even when the output of the switch is short circuited to
ground. The output voltage of a current limited regulator
may drop very quickly during short-circuit and pull the
supply pin of the LTC1154 below 3.5V before the shutdown
circuitry has had time to respond and remove drive from
the gate of the power MOSFET. A supply filter should be
Figure 7. Reverse Battery Protection
Figure 8. Supply Filter for Current Limited Supplies
IN
EN
STATUS
GND
V
S
DS
G
SD
LTC1154
MTP12N06
15V
0.05Ω
LTC1154 • F07
LOAD
10k
300Ω
5V
µP OR
CONTROL
LOGIC
+
10µF
10k
10k
120k
5V
10k
IN
EN
STATUS
GND
V
S
DS
G
SD
LTC1154
0.1Ω
IRLR024
SHORT-
CIRCUIT
0.1µF
100k
LTC1154 • F08
*20Ω
+
47µF*
+
10µF
1N4148
*SUPPLY FILTER COMPONENTS
+
100µF
5V/2A
REGULATOR
>7V
