LTC1154 datasheet(10/16 Pages) LINER

10 / 16 page

LTC1154

APPLICATI

I FOR ATIO

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 opera-

tion. (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

Figure 7. Reverse Battery Protection

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 15

µs.

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 8. Supply Filter for Current Limited Supplies

STATUS

GND

LTC1154

0.1

Ω

IRLR024

SHORT

CIRCUIT

0.1

µF

100k

LTC1154 • F08

*20

Ω

µF*

µF

1N4148

*SUPPLY FILTER COMPONENTS

100

µF

5V/2A

REGULATOR

>7V

Figure 6. Using a Speed-Up Diode

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 resistors 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 shut-

down circuitry has had time to respond and remove drive

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

STATUS

GND

LTC1154

100

µF

IRF530

15V

12V

0.036

Ω

LTC1154 • F06

0.01

µF

100k

1N4148

LOAD

STATUS

GND

LTC1154

MTP12N06

15V

12V

0.05

Ω

LTC1154 • F07

LOAD

10k

300

Ω

µP OR

CONTROL

LOGIC

µF

10k

120k

10k

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