10

LTC1153

S

APPLICATI

I FOR ATIO

The Y axis of the graph is normalized to one RC time

constant. The X axis is normalized to the set current. (The

set current is defined as the current required to develop

100mV across the drain sense resistor).

Note that the trip delay 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 S.O.A. information).

Using a Speed-Up Diode

Another way to reduce the trip delay time is to “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 trip

delay time. The drain sense resistor value is selected to

limit the maximum DC breaker current to 4A.

Figure 7. Reverse Battery Protection

Current Limited Power Supplies

The LTC1153 requires at least 3.5V at the supply pin to

ensure proper operation. It is therefore necessary that the

supply to the LTC1153 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 LTC1153 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

be added as shown in Figure 8 which holds the supply pin

of the LTC1153 high long enough for the over-current

shutdown circuitry to respond and fully discharge the

gate, i.e., break the circuit.

Reverse Battery Protection

The LTC1153 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. Since

the LTC1153 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.

Figure 8. Supply Filter for Current Limited Supplies

Figure 6. Using a Speed-Up Diode

IN

STATUS

GND

DS

G

SD

LTC1153

0.1

Ω

IRLR024

SHORT

CIRCUIT

0.1

µF

100k

1

µF

LTC1153 • F08

*20

Ω

+

*47

µF

+

10

µF

1N4148

*SUPPLY FILTER COMPONENTS

+

100

µF

5V/2A

REGULATOR

>7V

IN

STATUS

GND

DS

G

SD

LTC1153

0.22

µF

+

100

µF

IRF530

15V

12V

0.036

Ω

LTC1153 • F06

0.01

µF

1OOk

1N4148

LOAD

IN

STATUS

GND

DS

G

SD

LTC1153

0.47

µF

MTP12N06

15V

12V

0.05

Ω

LTC1153 • F07

LOAD

10k

300

Ω

5V

µP OR

CONTROL

LOGIC

+

10

µF

10k

10k

120k

5V