LTC1154
PPLICATI
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).
O U
W
U
A
S I FOR ATIO
12V
5V
+
10µF
120k
10k
10k
10k
IN
V
S
0.05Ω
5V
µP OR
CONTROL
LOGIC
EN
DS
G
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 opera-
tion. (See MOSFET data sheet for further information).
LTC1154
STATUS
GND
MTP12N06
15V
SD
LOAD
10k
300Ω
Using a Speed-Up Diode
LTC1154 • F07
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
Figure 7. Reverse Battery Protection
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 resistors in series with the input and status pins.
Current Limited Power Supplies
12V
+
100µF
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,evenwhentheoutputoftheswitchisshortcircuited
toground.Theoutputvoltageofacurrentlimitedregulator
may drop very quickly during short circuit and pull the
supply pin of the LTC1154 below 3.5V before the shut-
down circuitry has had time to respond and remove drive
from the gate of the power MOSFET. A supply filter should
1N4148
IN
V
S
0.036Ω
0.01µF
100k
EN
DS
G
LTC1154
STATUS
IRF530
15V
GND
SD
LOAD
LTC1154 • F06
5V/2A
REGULATOR
Figure 6. Using a Speed-Up Diode
>7V
+
+
*20Ω
10µF
0.1Ω
100µF
+
1N4148
100k
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.
47µF*
IN
V
S
0.1µF
EN
DS
G
LTC1154
IRLR024
STATUS
Reverse Battery Protection
SHORT
CIRCUIT
GND
SD
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.
LTC1154 • F08
*SUPPLY FILTER COMPONENTS
Figure 8. Supply Filter for Current Limited Supplies
10
Linear [ Linear ]
