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AAT4250IGV-T1

Slew Rate Controlled Load Switch

Analog Technology Inc 

AAT4250

Slew Rate Controlled Load Switch

Applications Information

Input Capacitor

Typically a 1µF or larger capacitor is recommend-

ed for CIN in most applications. A CIN capacitor is

not required for basic operation, however, it is use-

ful in preventing load transients from affecting up

stream circuits. CIN should be located as close to

the device VIN pin as practically possible. Ceramic,

tantalum or aluminum electrolytic capacitors may

be selected for CIN. There is no specific capacitor

ESR requirement for CIN. However, for higher cur-

rent operation, ceramic capacitors are recom-

mended for CIN due to their inherent capability over

tantalum capacitors to withstand input current

surges from low impedance sources such as bat-

teries in portable devices.

Output Capacitor

For proper slew operation, a 0.1µF capacitor or

greater between VOUT and GND is required.

Likewise, with the output capacitor, there is no spe-

cific capacitor ESR requirement. If desired, COUT

maybe increased without limit to accommodate any

load transient condition without adversely affecting

the slew rate.

Enable Function

The AAT4250 features an enable / disable function.

This pin (ON) is active high and is compatible with

TTL or CMOS logic. To assure the load switch will

turn on, the ON control level must be greater than

2.0 volts. The load switch will go into shutdown

mode when the voltage on the ON pin falls below

0.8 volts. When the load switch is in shutdown

mode, the OUT pin is tristated, and quiescent cur-

rent drops to leakage levels below 1µA.

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.

8

In applications where there is a possibility of VOUT

exceeding VIN for brief periods of time during nor-

mal operation, the use of a larger value CIN capaci-

tor 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 schot-

tky diode from VIN to VOUT (connecting the cathode

to VIN and anode to VOUT). The Schottky diode for-

ward voltage should be less then 0.45 volts.

Thermal Considerations and High

Output Current Applications

The AAT4250 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.

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] / Θ JA

Constants for the AAT4250 are maximum junction

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

resistance, ΘJA = 150°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) = 267mW. At TA =

25°C, PD(MAX) = 667mW.

The maximum continuous output current for the

AAT4250 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 tempera-

ture RDS by the RDS temperature coefficient. The

temperature coefficient (TC) is 2800ppm/°C.

Therefore, at 125°C

RDS(MAX) = RDS(25°C) × (1 + TC × ∆T)

RDS(MAX) = 175mΩ × (1 + .002800 × (125°C - 25°C))

RDS(MAX) = 224mΩ

4250.2001.12.0.94

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