AAT4282B Datasheet, PDF (10/13 Pages) Skyworks Solutions Inc. – Slew Rate Controlled Load Switch

maximum room temperature R_DS by the R_DS temperature
coefficient. The temperature coefficient (TC) is
2800ppm/°C. Therefore, at 125°C:
R_DS(MAX) = R_DS(25°C) · (1 + TC · T)
R_DS(MAX) = 130m· (1 + 0.002800 · (125°C - 25°C))
R_DS(MAX) = 166.4m
For maximum current, refer to the following equation:
I_OUT(MAX) <
P_D(MAX)
R_DS
For example, if V_IN = 5V, R_DS(MAX) = 166.4m, and T_A =
25°C, I_OUT(MAX) = 2.93A. If the output load current were
to exceed 2.93A 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 AAT4282B. 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 described in the High Peak Current Applications
section of this datasheet.
High Peak Output Current Applications
Some applications require the load switch to operate at a
continuous nominal current level with short duration,
high-current peaks. Refer to the I_DM specification in the
Absolute Maximum Ratings table to ensure the AAT4282B’s
maximum pulsed current rating is not exceeded. The duty
cycle for both output current levels must be taken into
account. To do so, first calculate the power dissipation at
the nominal continuous current level, and then add the
additional power dissipation due to the short duration,
high-current peak scaled by the duty factor. For example,
a 4V system using an AAT4282B which has channel A
operates at a continuous 1A load current level, and chan-
nel B operates at a continuous 100mA load current level
and has short 3A current peaks, as in a GSM application.
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 3A for 12.5% of the period. Since the
Electrical Characteristics do not report R_DS(MAX) for 4V
DATA SHEET
AAT4282B
Slew Rate Controlled Load Switch
operation, it must be approximated by consulting the
chart of R_DS(ON) vs. V_IN. The R_DS reported for 5V at 100mA
and 3A can be scaled by the ratio seen in the chart to
derive the R_DS for 4V V_IN at 25°C: 130m · 72m/67m
= 139.7m. De-rated for temperature: 139.7m · (1 +
0.002800 · (125°C - 25°C)) = 178.8mΩ.
For channel A, the power dissipation for a continuous 1A
load is calculated as follows:
P_D(CHA) = I_OUT2 · R_DS = (1A)² · 178.8m = 178.8mW
For channel B, the power dissipation for 100mA load is
calculated as follows:
P_D(MAX) = I_OUT2 · R_DS
P_D(100mA) = (100mA)² · 178.8m
P_D(100mA) = 1.79mW
P_D(87.5%D/C) = %DC · P_D(100mA)
P_D(87.5%D/C) = 0.875 · 1.79mW
P_D(87.5%D/C) = 1.57mW
The power dissipation for 100mA load at 87.5% duty
cycle is 1.57mW. Now the power dissipation for the
remaining 12.5% of the duty cycle at 3A is calculated:
P_D(MAX) = I_OUT2 · R_DS
P_D(3A) = (3A)² · 178.8m
P_D(3A) = 1609mW
P_D(12.5%D/C) = %DC · P_D(3A)
P_D(12.5%D/C) = 0.125 · 1609mW
P_D(12.5%D/C) = 201.1mW
Finally, the total power dissipation for channels A and B
is determined as follows:
P_D(total) = P_D(CHA) + P_D(100mA) + P_D(3A)
P_D(total) = 178.8mW + 1.57mW + 201.1mW
P_D(total) = 381mW
The maximum power dissipation for the AAT4282B oper-
ating at an ambient temperature of 85°C is 381mW. The
device in this example will have a total power dissipation
of 541mW. This is well within the thermal limits for safe
operation of the device; in fact, at 85°C, the AAT4282B
will handle a 3A pulse for up to 22 duty cycle. At lower
ambient temperatures, the duty cycle can be further
increased.
Skyworks Solutions, Inc. • Phone [781] 376-3000 • Fax [781] 376-3100 • [e-mail] • www.skyworksinc.com
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201859B • Skyworks Proprietary Information • Products and Product Information are Subject to Change Without Notice. • May 14, 2012