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AAT4250 Datasheet(PDF) 9 Page - Advanced Analogic Technologies |
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AAT4250 Datasheet(HTML) 9 Page - Advanced Analogic Technologies |
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9 / 12 page  For maximum current, refer to the following equation: IOUT(MAX) < ( PD(MAX) / RDS) 1/2 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 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. 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 IDM 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 RDS MAX for 4 volts operation, it must be calculated approximated by consulting the chart of RDSON vs. VIN. The Rds reported for 5 volt RDS can be scaled by the ratio seen in the chart to derive the Rds for 4 volt VIN: 175m Ω x 120mΩ/115mΩ = 183mΩ. Derated for temperature: 183m Ω x (1 + .002800 x (125°C - 25°C)) = 235m Ω . The power dissipation for a 100mA load is calculated as follows: PD(MAX) = I 2 OUT x RDS PD(100mA) = (100mA) 2 x 235m Ω PD(100mA) = 2.35mW PD(87.5%D/C) = %DC x PD(100mA) PD(87.5%D/C) = 0.875 x 2.35mW PD(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: PD(MAX) = I 2 OUT x RDS PD(2A) = (2A) 2 x 235m Ω PD(2A) = 940mW PD(12.5%D/C) = %DC x PD(2A) PD(12.5%D/C) = 0.125 x 940mW PD(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. PD(total) = PD(100mA) + PD(2A) PD(total) = 2.1mW + 117.5mW PD(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. AAT4250 Slew Rate Controlled Load Switch 4250.2001.12.0.94 9 |
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Similar Description - AAT4250 |
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