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AAT4250

Slew Rate Controlled Load Switch

Advanced Analogic Technologies 

AAT4250

Slew Rate Controlled Load Switch

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

2.0V. The load switch will go into shutdown mode

when the voltage on the ON pin falls below 0.8V.

When the load switch is in shutdown mode, the

OUT pin is tri-stated, and quiescent current 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, maintaining 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, possibly damaging the load switch.

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

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 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 must be taken into account.

The following discussions will assume the load

switch is mounted on a printed circuit board utilizing

the minimum recommended footprint as stated in

the Printed Circuit Board Layout Recommendations

section of this datasheet.

4250.2006.03.1.3

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

mum package power dissipation can be deter-

mined by the following equation:

PD(MAX)

=

TJ(MAX) -

θJA

TA

Constants for the AAT4250 are maximum junction

temperature (TJ(MAX) = 125°C) and package ther-

mal resistance (ΘJA = 150°C/W). Worst case con-

ditions are calculated at the maximum operating

temperature, 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 + 0.002800 × (125°C - 25°C))

RDS(MAX) = 224mΩ

For maximum current, refer to the following equation:

1

IOUT(MAX) <

⎛ PD(MAX)⎞ 2

⎝ RDS ⎠

For example, if VIN = 5V, RDS(MAX) = 224mΩ, and TA

= 25°C, IOUT(MAX) = 1.7A. If the output load current

were to exceed 1.7A or if the ambient temperature

were to increase, the internal die temperature

would increase and the device would be damaged.

Higher peak currents can be obtained with the

AAT4250. To accomplish this, the device thermal

resistance must be reduced by increasing the heat

sink area or by operating the load switch in a duty-

cycle manner. Duty cycles with peaks less than

2ms in duration can be considered using the

method below.

9

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