7

LTC1255

APPLICATIO S I FOR ATIO

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 10

µs 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 (< 1

µF) 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.

DS1

1/2 LTC1255

G1

GND

IN1

IRFZ24

12V

18V

+

10

µF

18

Ω

0.036

Ω

≤ 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 10

µs 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

DS1

1/2 LTC1255

G1

GND

IN1

IRFZ24

12V

12V

+

100

µF

12V, 1A

SOLENOID

0.036

Ω

1N5400

LTC1255 • F02

DS1

1/2 LTC1255

G1

GND

IN1

MTP3055E

12V

15V

+

470

µF

0.036

Ω

LTC1255 • F03

0.01

100k

D1

1N4148

R1

100k

R2

100k

C1

0.33

µF

+

100

µF