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전자부품 데이터시트 검색엔진 |
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전자부품 데이터시트 검색엔진 |
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LTC3246 데이터시트(HTML) 16 Page - Analog Devices |
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LTC3246 데이터시트(HTML) 16 Page - Analog Devices |
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LTC3246 16 3246fa For more information www.linear.com/LTC3246 APPLICATIONS INFORMATION siderably. Poor board layout and failure to connect the die pad (Pin 17) to a large ground plane can result in thermal junction to ambient impedance well in excess of 40°C/W. It is also possible to get thermal rates less than 40°C/W with good airflow over the part and PC board. Because of the wide input operating range, it is possible to exceed the specified operating junction temperature and even reach thermal shutdown (175°C typ). Figure 6 and Figure 7 show the available output current vs ambi- ent temperature to ensure the 150°C operating junction temperature is not exceeded. Thefiguresassumeworst-caseoperatingconditionsanda thermal impedance of 40°C/W. It is always safe to operate under the line shown on the graph. Operation above the line is conditional and is the responsibility of the user to calculate worst-case operating conditions (temperature and power) to make sure the part does not exceed the 150°C operating junction temperature for extended pe- riods of time. The 2:1 Step-Down Charge Pump Operation, 1:1 Step- Down Charge Pump Operation, and 1:2 Step-Up Charge PumpOperationsectionsprovideequationsforcalculating power dissipation (PD) in each mode. For example, if it is determined that the maximum power dissipation (PD) is 1.2W under normal operation, then the junction to ambient temperature rise will be: TJA = 1.2W • 40°C/W = 48°C Thus, the ambient temperature under this condition can- not exceed 102°C if the junction temperature is to remain below 150°C, and, if the ambient temperature exceeds about 127°C, the device will cycle in and out of the thermal shutdown. Every application will have a slightly different thermal rise than the specified 40°C/W, especially applications with good airflow. Calculating the actual thermal rate for a specific application circuit is too complex to be presented here, but the thermal rate can be measured in application. This is done by first taking the final application circuit and enabling the LTC3246 under a known power dissipation (PD) and raising the ambient temperature slowly until the LTC3246 shuts down. Note this temperature as T1. Now, remove the load from the part and raise the ambi- ent temperature slowly until the LTC3246 shuts down again. Note this temperature as T2. The thermal rate can be calculated as: JA = PD/(T2 – T1) Another method for determining maximum safe operating temperature in application is to configure the LTC3246 to operate under the worst case operating power dissipa- tion. Then slowly raise the ambient temperature until the LTC3246 shuts down. At this point the LTC3246 junction temperature will be about 175°C, so simply subtract 25°C from the shutdown temperature and this is the safe operating temperature for the application. Figure6. AMBIENT TEMPERATURE (°C) 0 0.5 0.4 0.1 0.3 0.2 0.0 125 75 3246 F06 150 100 50 25 2.7V < VIN < 22V θJA = 40°C/W CONDITIONAL OPERATION SAFE OPERATION 5V Output Operation vs Ambient Temperature Figure7. AMBIENT TEMPERATURE (°C) 0 0.5 0.4 0.1 0.3 0.2 0.0 125 75 3246 F07 150 100 50 25 2.7V < VIN < 15V θJA = 40°C/W CONDITIONAL OPERATION SAFE OPERATION 3.3V Output Operation vs Ambient Temperature |
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