REV. A

ADM869L

–7–

FUNCTIONAL DESCRIPTION

The ADM869L comprises a high-current P-channel switch con-

trolled by an active-low logic input

ON (Pin 7). When ON is

low, the internal circuitry of the ADM869 is powered up and the

output of the current-limit amplifier is low, providing gate drive

to the switching FET. When

ON is high, the internal circuitry is

powered down and the current consumption is typically 100 nA.

It should be noted that the ADM869L is not a bidirectional

CURRENT LIMIT

When the switch is turned on, a smaller mirror switch passes a

maintains this relationship by keeping the drain of the mirror

FET at the same voltage as the main FET, and drives the mirror

current through the current-limit resistor RSET, which is

connected between the noninverting input of the current limit

amplifier and ground. An on-chip bandgap reference of 1.24 V

is connected to the inverting input of the current-limit amplifier.

When the load current exceeds the preset limit, the voltage

current-limit amplifier rises, reducing the gate drive to the FETs.

1.047 mA = 1.184 k

current so please refer to TPC 6.

ADM869L

SET

1.24V

955

Figure 1. Setting the Current Limit

THERMAL SHUTDOWN

The thermal shutdown operates when the die temperature

exceeds 135

°C, turning off the switch. The thermal shutdown

circuit has built-in hysteresis of 10

°C, so the switch will not turn

on again until the die temperature falls to 125

°C. If the fault

condition is not removed, the switch will pulse on and off as the

temperature cycles between these limits.

FAULT OUTPUT

If either the current limit or the thermal shutdown is activated,

FAULT will pull low. This is an open-drain output and requires

a pull-up resistor of between 10 k

Ω and 100 kΩ. Several FAULT

outputs may be wire-OR’d to form a common interrupt line, as

shown in Figure 2 or

FAULT may be wire-OR’d to an existing

interrupt line that has a resistive pull-up.

ADM869L

FAULT

INT

ADM869L

FAULT

ADM869L

FAULT

Figure 2. Wire Or’ing

FAULT Outputs

During startup, the

FAULT output goes low for the turn-on time

plus 50

µs.

APPLICATIONS INFORMATION

INPUT FILTERING

To prevent the input voltage being pulled below the minimum

operating voltage under transient short-circuit conditions, before

the current limit has had time to operate, a reservoir capacitor

should be connected from IN to GND. This does not need to be

large, but should have a low ESR. A 22

µF ceramic type is

suitable. Larger values will reduce the voltage drop still further.

OUTPUT CAPACITANCE

A 0.1

µF capacitor should be connected between OUT and

GND to prevent the back e.m.f. of parasitic inductance from

pulling OUT below ground during turn-off. For Universal Serial

µF. This

causes the output rise and fall times to be longer, as shown in

the typical operating characteristics, but does not affect the

turn-off time of the ADM869L itself.

LAYOUT CONSIDERATIONS

Printed circuit board tracks to and from the ADM869L should be

as thick and as short as possible to minimize parasitic inductance

and take full advantage of the fast response time of the switch. It is

recommended that all input lines be connected together, close to

the device. This ensures equal current distribution in all legs. If

this is not possible, then all traces should be of equal width and

length. The same rules apply for all output lines. Input and output

capacitors should be placed as close to the device as possible (less

than 5 mm).

THERMAL CONSIDERATIONS

Under normal operating conditions, the worst-case power dissi-

pation will be 518 mW with the highest specified on resistance

and 3 V supply (W = 2.4 A

× 0.9 V). The package is capable of

handling and dissipating this power, but heat dissipation can

further be improved by providing a large area of copper in con-

tact with the device pins, particularly IN and OUT.