7
LTC1255
APPLICATIO S I FOR ATIO
WUU U
MOSFET AND LOAD PROTECTION
The LTC1255 protects the power MOSFET switch by
removing drive from the gate as soon as an overcurrent
condition is detected. Resistive and inductive loads can be
protected with no external time delay in series with the
drain sense pin. Lamp loads, however, require that the
overcurrent protection be delayed long enough to start the
lamp but short enough to ensure the safety of the MOSFET.
Resistive Loads
Loads that are primarily resistive should be protected with
as short a delay as possible to minimize the amount of time
that the MOSFET is subjected to an overload condition.
The drain sense circuitry has a built-in delay of approxi-
mately 10µs to eliminate false triggering by power supply
or load transient conditions. This delay is sufficient to
“mask” short load current transients and the starting of a
small capacitor (<1µF) in parallel with the load. The drain
sense pin can therefore be connected directly to the drain
current sense resistor as shown in Figure 1.
V
S
DS1
1/2 LTC1255
G1
GND
IN1
IRFZ24
12V
18V
+
10µF
R
LOAD
18Ω
R
SENSE
0.036Ω
C
LOAD
≤ 1µF
LTC1255 • F01
Figure 1. Protecting Resistive Loads
Inductive Loads
Loads that are primarily inductive, such as relays, sole-
noids and stepper motor windings, should be protected
with as short a delay as possible to minimize the amount
of time that the MOSFET is subjected to an overload
condition. The built-in 10µs delay will ensure that the
overcurrent protection is not false triggered by a supply or
load transient. No external delay components are required
as shown in Figure 2.
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 rating
should be connected across the load, as shown in
Figure 2, to safely divert the stored energy.
Figure 2. Protecting Inductive Loads
Capacitive Loads
Large capacitive loads, such as complex electrical sys-
tems with large bypass capacitors, should be powered
using the circuit shown in Figure 3. The gate drive to the
power MOSFET is passed through an RC delay network,
R1 and C1, which greatly reduces the turn-on ramp rate of
the switch. And since the MOSFET source voltage follows
the gate voltage, the load is powered smoothly and slowly
from ground. This dramatically reduces the startup cur-
rent flowing into the supply capacitor(s) which, in turn,
reduces supply transients and allows for slower activation
Figure 3. Powering Large Capacitive Loads
V
S
DS1
1/2 LTC1255
G1
GND
IN1
IRFZ24
12V
12V
+
100µF
12V, 1A
SOLENOID
R
SENSE
0.036Ω
1N5400
LTC1255 • F02
V
S
DS1
1/2 LTC1255
G1
GND
IN1
MTP3055E
12V
15V
+
470µF
R
SENSE
0.036Ω
LTC1255 • F03
C
DELAY
0.01µF
R
DELAY
100k
D1
1N4148
R1
100k
R2
100k
C1
0.33µF
+
C
LOAD
100µF
