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FAN1084T 데이터시트(PDF) 7 Page - Fairchild Semiconductor |
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FAN1084T 데이터시트(HTML) 7 Page - Fairchild Semiconductor |
7 / 13 page PRODUCT SPECIFICATION FAN1084 REV. 1.0.8 11/10/03 7 The current out of the adjust pin adds to the current from R1. Its output voltage contribution is small and only needs consid- eration when a very precise output voltage setting is required. Figure 3. Connection for Best Load Regulation Load Regulation It is not possible to provide true remote load sensing because the FAN1084 series are three-terminal devices. Load regula- tion is limited by the resistance of the wire connecting the reg- ulator to the load. Load regulation per the data sheet specification is measured at the bottom of the package. For fixed voltage devices, negative side sensing is a true Kelvin connection with the ground pin of the device returned to the negative side of the load. This is illustrated in Figure 4. Figure 4. Connection for Best Load Regulation For adjustable voltage devices, negative side sensing is a true Kelvin connection with the bottom of the output divider returned to the negative side of the load. The best load regula- tion is obtained when the top of the resistor divider R1 connects directly to the regulator output and not to the load. Figure 5 illustrates this point. If R1 connects to the load, then the effective resistance between the regulator and the load would be: RP X (1 + R2/R1), RP = Parasitic line Resistance The connection shown in Figure 5 does not multiply RP by the divider ration. As an example, RP is about four milliohms per foot with 16-gauge wire. This translates to 4mV per foot at 1A load current. At higher load currents, this drop represents a significant percentage of the overall regulation. It is important to keep the positive lead between the regulator and the load as short as possible and to use large wire or PC board traces. Figure 5. Connection for Best Load Regulation Thermal Conditions The FAN1084 series protect themselves under overload con- ditions with internal power and thermal limiting circuitry. However, for normal continuous load conditions, do not exceed maximum junction temperature ratings. It is impor- tant to consider all sources of thermal resistance from junc- tion-to-ambient. These sources include the junction-to-case resistance, the case-to-heatsink interface resistance, and the heat sink resistance. Thermal resistance specifications have been developed to more accurately reflect device tempera- ture and ensure safe operating temperatures. The electrical characteristics section provides a separate thermal resistance and maximum junction temperature for both the control cir- cuitry and the power transistor. Calculate the maximum junc- tion temperature for both sections to ensure that both thermal limits are met. For example, look at using a FAN1084T to generate 4.5A @ 1.5V from a 3.3V source (3.2V to 3.6V). Assumptions •VIN = 3.4V worst case •VOUT = 1.475V worst case •IOUT = 4.5A continuous •TA = 60°C • θCase-to-Ambient = 5°C/W (assuming both a heatsink and a thermally conductive material) The power dissipation in this application is: PD = (VIN – VOUT) * (IOUT) = (3.6 – 1.475) * (4.5) = 9.6W From the specification table: TJ = TA + (PD) * (θCase-to-Ambient + θJC) = 60 + (9.6) * (5 + 3) = 137 °C The junction temperature is below the maximum thermal limit. FAN1084 ADJ C2 22 µF VOUT + C1 10 µF IADJ 35 µA + IN OUT VIN R1 R2 VREF FAN1084-1.5 GND RL RP Parasitic Line Resistance IN OUT VIN FAN1084 ADJ RL R1* R2* *Connect R1 to case Connect R2 to load RP Parasitic Line Resistance IN OUT VIN |
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