LTC4441 datasheet(10/18 Pages) LINER

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LTC1154

1154fb

APPLICATIONS INFORMATION

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).

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 operation. (See

MOSFET data sheet for further information).

Using a Speed-Up Diode

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 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.

Reverse Battery Protection

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.

Figure 6. Using a Speed-Up Diode

STATUS

GND

LTC1154

100μF

IRF530

15V

12V

0.036Ω

LTC1154 • F06

0.01μF

100k

1N4148

LOAD

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 resis-

tors in series with the input and status pins.

Current Limited Power Supplies

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,

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 LTC1154 below 3.5V before the shutdown

circuitry has had time to respond and remove drive from

the gate of the power MOSFET. A supply ﬁlter should be

Figure 7. Reverse Battery Protection

Figure 8. Supply Filter for Current Limited Supplies

STATUS

GND

LTC1154

MTP12N06

15V

12V

0.05Ω

LTC1154 • F07

LOAD

10k

300Ω

μP OR

CONTROL

LOGIC

10μF

10k

120k

10k

STATUS

GND

LTC1154

0.1Ω

IRLR024

SHORT-

CIRCUIT

0.1μF

100k

LTC1154 • F08

*20Ω

47μF*

10μF

1N4148

*SUPPLY FILTER COMPONENTS

100μF

5V/2A

REGULATOR

>7V

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