AAT4250
Slew Rate Controlled Load Switch
For maximum current, refer to the following equation:
I
OUT(MAX)
< ( P
D(MAX)
/ R
DS
)
1/2
For example, if V
IN
= 5V, R
DS(MAX)
=224mΩ and T
A
= 25°C, I
OUT(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 will
increase, and the device will 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.
by consulting the chart of R
DSON
vs. V
IN
. The Rds
reported for 5 volt R
DS
can be scaled by the ratio
seen in the chart to derive the Rds for 4 volt V
IN
:
175mΩ x 120mΩ/115mΩ = 183mΩ. Derated for
temperature: 183mΩ x (1 + .002800 x (125°C -
25°C)) = 235mΩ . The power dissipation for a
100mA load is calculated as follows:
P
D(MAX)
= I
2
OUT
x R
DS
P
D(100mA)
= (100mA)
2
x 235mΩ
P
D(100mA)
= 2.35mW
P
D(87.5%D/C)
= %DC x P
D(100mA)
P
D(87.5%D/C)
= 0.875 x 2.35mW
P
D(87.5%D/C)
= 2.1mW
The power dissipation for 100mA load at 87.5%
duty cycle is 2.1mW. Now the power dissipation for
the remaining 12.5% of the duty cycle at 2A is cal-
culated:
P
D(MAX)
= I
2
OUT
x R
DS
P
D(2A)
= (2A)
2
x 235mΩ
P
D(2A)
= 940mW
P
D(12.5%D/C)
= %DC x P
D(2A)
P
D(12.5%D/C)
= 0.125 x 940mW
P
D(12.5%D/C)
= 117.5mW
The power dissipation for 2A load at 12.5% duty
cycle is 117mW. Finally, the two power figures are
summed to determine the total true power dissipa-
tion under the varied load.
P
D(total)
= P
D(100mA)
+ P
D(2A)
P
D(total)
= 2.1mW + 117.5mW
P
D(total)
= 120mW
The maximum power dissipation for the AAT4250
operating at an ambient temperature of 85°C is
267mW. The device in this example will have a
total power dissipation of 120mW. This is well with
in the thermal limits for safe operation of the
device, in fact, at 85°C, the AAT4250 will handle a
2A pulse for up to 28% duty cycle. At lower ambi-
ent temperatures the duty cycle can be further
increased.
High Peak Output Current Applications
Some applications require the load switch to oper-
ate at a continuous nominal current level with short
duration high current peaks. Refer to the I
DM
spec-
ification in the Absolute Maximum table to ensure
the AAT 4250’s maximum pulsed current rating is
not exceeded. The duty cycle for both output cur-
rent levels must be taken into account. To do so,
first calculate the power dissipation at the nominal
continuous current level, and then add in the addi-
tional power dissipation due to the short duration
high current peak scaled by the duty factor.
For example, a 4V system using an AAT4250 oper-
ates at a continuous 100mA load current level and
has short 2A current peaks, as in a GSM applica-
tion. The current peak occurs for 576µs out of a
4.61ms period.
First, the current duty cycle is calculated:
% Peak Duty Cycle: X/100 = 576µs/4.61ms
% Peak Duty Cycle = 12.5%
The load current is 100mA for 87.5% of the 4.61ms
period and 2A for 12.5% of the period. Since the
Electrical Characteristics do not report R
DS MAX
for 4
volts operation, it must be calculated approximated
4250.2001.12.0.94
9
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
