AAT4280A_0511 Datasheet, PDF (11/14 Pages) Advanced Analogic Technologies – Slew Rate Controlled Load Switch

AAT4280A
Slew Rate Controlled Load Switch
sipation and the R_DS of the MOSFET at T_J(MAX).
The maximum R_DS of the MOSFET at T_J(MAX) is cal-
culated by increasing the maximum room tempera-
ture R_DS by the R_DS temperature coefficient. The
temperature coefficient (T_C) is 2800ppm/°C.
Therefore,
MAX R_DS125°C = R_DS25°C × (1 + T_C × ∆T)
MAX R_DS125°C = 120mΩ × (1 + 0.0028 ×
(125°C - 25°C)) = 154mΩ
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) = 154mΩ and T_A
= 25°C, I_OUT(MAX) = 2.3A. If the output load current
were to exceed 2.3A 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
AAT4280A. 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.
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. The duty cycle for
both output current levels must be taken into
account. To do so, first calculate the power dissi-
pation at the nominal continuous current level, and
then add in the additional power dissipation due to
the short duration, high-current peak scaled by the
duty factor.
For example, a 4V system using an AAT4280A
operates at a continuous 100mA load current level
and has short 2A current peaks, as in a GSM appli-
cation. 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
4V operation, it must be calculated approximately
by consulting the chart of R_DS(ON) vs. V_IN. The R_DS
reported for 5V can be scaled by the ratio seen in
the chart to derive the R_DS for a 4V V_IN: 120mΩ ×
87mΩ /80mΩ = 130mΩ. De-rated for temperature:
130mΩ x (1 + 0.0028 × (125°C -25°C)) = 166mΩ.
The power dissipation for a 100mA load is calculat-
ed as follows:
P_D(MAX) = I_OUT² × R_DS
P_D(100mA) = (100mA)² × 166mΩ
P_D(100mA) = 1.66mW
P_D(87.5%D/C) = %DC × P_D(100mA)
P_D(87.5%D/C) = 0.875 × 1.66mW
P_D(87.5%D/C) = 1.45mW
The power dissipation for 100mA load at 87.5%
duty cycle is 1.45mW. Now the power dissipation
for the remaining 12.5% of the duty cycle at 2A is
calculated:
P_D(MAX) = I_OUT² × R_DS
P_D(2A) = (2A)² × 166mΩ
P_D(2A) = 664mW
P_D(12.5%D/C) = %DC × P_D(2A)
P_D(12.5%D/C) = 0.125 × 664mW
P_D(12.5%D/C) = 83mW
The power dissipation for 2A load at 12.5% duty
cycle is 83mW. 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) = 1.45mW + 83mW
P_D(total) = 84.5mW
The maximum power dissipation for the AAT4280A
operating at an ambient temperature of 85°C is
333mW. The device in this example will have a
total power dissipation of 84.5mW. This is well
within the thermal limits for safe operation of the
device; in fact, at 85°C, the AAT4280A will handle
a 2A pulse for up to 50% duty cycle. At lower ambi-
ent temperatures, the duty cycle can be further
increased.
4280A.2005.11.1.3
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