LTC1155
8
1155fb
For more information www.linear.com/LTC1155
APPLICATIONS INFORMATION
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 4A. Above 28A, the delay time drops to 10µs.
Switched Supply Applications
Large inductive loads, such as solenoids, relays and mo-
tors store energy which must be directed back to either
the power supply
or to ground when the supply voltage is
interrupted (see Figure 4). In normal operation, when the
switch is turned OFF, the energy stored in the inductor is
harmlessly absorbed by the MOSFET; i.e., the current flows
out of the supply through the MOSFET until the inductor
current falls to zero.
IRLZ34
LOAD
LTC1155
GND
GND
G1
DS1
V
S
IN1
V
S
= 5.0V
C
DLY
0.22µF
R
SEN
0.025Ω
R
DLY
270k
D1
1N4148
Figure 4. Switched Supply
Figure 3. Using a Speed-Up Diode
IRLZ34
L
LOAD
LTC1155
GND
GND
G1
DS1
V
S
IN1
C
DLY
R
SEN
0.025Ω
R
DLY
C
S
+
+
If the MOSFET is turned ON and the power supply (battery)
removed, the inductor current is delivered by the supply
capacitor. The supply capacitor must be large enough to
deliver the energy demanded by the discharging inductor.
If the storage capacitor is too small, the supply lead of
the LTC1155 may be pulled below ground, permanently
destroying the device.
Consider the case of a load inductance
of 1mH which
is supporting 3A when the 6V power supply connection
is interrupted. A supply capacitor of at least 250µF is
required to prevent the supply lead of the LTC1155 from
being pulled below ground (along with any other circuitry
tied to the supply).
Any wire between the power MOSFET source and the load
will add a small amount of parasitic inductance in series
with
the load (approximately 0.4µH/foot). Bypass the power
supply lead of the LTC1155 with a minimum of 10µF to
ensure that this parasitic load inductance is discharged
safely, even if the load is otherwise resistive.
Large Inductive Loads
Large inductive loads (>0.1mH) may require diodes con-
nected directly across the inductor to safely divert the
stored energy to ground. Many inductive loads have these
diodes included. If
not, a diode of the proper current rat-
ing should be connected across the load to safely divert
the stored energy.
Reverse-Battery Protection
The LTC1155 can be protected against reverse-battery
conditions by connecting a resistor in series with the
ground lead as shown in Figure 5. The resistor limits the
supply current to less than 50mA with –12V applied. Since
the LTC1155 draws very little current
while in normal
operation, the drop across the ground resistor is minimal.
The TTL or CMOS driving logic is protected against
reverse-battery conditions by the 100k input current
limiting resistor. The addition of 100k resistance in series
with the input pin will not affect the turn ON and turn OFF
times which are dominated by the controlled gate charge
and discharge periods.
