AAT4280_06 datasheet(10/14 Pages) ANALOGICTECH

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AAT4280

Slew Rate Controlled Load Switch

4280.2006.11.1.4

Applications Information

Input Capacitor

A 1µF or larger capacitor is typically recommended

for CIN in most applications. A CIN capacitor is not

required for basic operation. However, CIN is use-

ful in preventing load transients from affecting

upstream circuits. CIN should be located as close

to the device VIN pin as practically possible.

Ceramic, tantalum, or aluminum electrolytic capac-

itors may be selected for CIN. There is no specific

capacitor ESR requirement for CIN. However, for

higher current operation, ceramic capacitors are

recommended for CIN due to their inherent capabil-

ity over tantalum capacitors to withstand input cur-

rent surges from low impedance sources, such as

batteries in portable devices.

Output Capacitor

For proper slew operation, a 0.1µF capacitor or

greater between VOUT and GND is recommended.

The output capacitor has no specific capacitor type

or ESR requirement.

If desired, COUT may be

increased without limit to accommodate any load

transient condition without adversely affecting the

device turn-on slew rate time.

Enable Function

The AAT4280 features an enable / disable function.

This pin (ON/OFF) is compatible with both TTL or

CMOS logic.

Reverse Output-to-Input Voltage

Conditions and Protection

Under normal operating conditions, a parasitic

diode exists between the output and input of the

load switch.

The input voltage should always

remain greater than the output load voltage, main-

taining a reverse bias on the internal parasitic

diode.

Conditions where VOUT might exceed VIN

should be avoided since this would forward bias

the internal parasitic diode and allow excessive

current flow into the VOUT pin and possibly damage

the load switch.

In applications where there is a possibility of VOUT

exceeding VIN for brief periods of time during normal

operation, the use of a larger value CIN capacitor is

highly recommended. A larger value of CIN with

respect to COUT will effect a slower CIN decay rate

during shutdown, thus preventing VOUT from

exceeding VIN. In applications where there is a

greater danger of VOUT exceeding VIN for extended

periods of time, it is recommended to place a

Schottky diode from VIN to VOUT (connecting the

cathode to VIN and anode to VOUT). The Schottky

diode forward voltage should be less than 0.45V.

Thermal Considerations and

High Output Current Applications

The AAT4280 is designed to deliver a continuous

output load current. The limiting characteristic for

maximum safe operating output load current is

package power dissipation. In order to obtain high

operating currents, careful device layout and circuit

operating conditions need to be taken into account.

The following discussions will assume the load

switch is mounted on a printed circuit board utiliz-

ing the minimum recommended footprint, as stated

in the Layout Considerations section of this

datasheet.

At any given ambient temperature (TA), the maxi-

mum package power dissipation can be deter-

mined by the following equation:

PD(MAX) = [TJ(MAX) - TA] / ΘJA

Constants for the AAT4280 are maximum junction

temperature, TJ(MAX) = 125°C, and package thermal

resistance,

ΘJA = 120°C/W. Worst case conditions

are calculated at the maximum operating tempera-

ture where TA = 85°C. Typical conditions are cal-

culated under normal ambient conditions where TA

= 25°C. At TA = 85°C, PD(MAX) = 333mW. At TA =

25°C, PD(MAX) = 833mW.

The maximum continuous output current for the

AAT4280 is a function of the package power dissi-

pation and the RDS of the MOSFET at TJ(MAX). The

maximum RDS of the MOSFET at TJ(MAX) is calcu-

lated by increasing the maximum room temperature

RDS by the RDS temperature coefficient. The tem-

perature coefficient (TC) is 2800ppm/°C. Therefore,

MAX RDS125°C = RDS25°C · (1 + TC · ΔT)

MAX RDS125°C = 120mΩ · (1 + 0.0028 ·

(125°C - 25°C)) = 154m

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