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FAN1587ADX Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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FAN1587ADX Datasheet(HTML) 8 Page - Fairchild Semiconductor |
8 / 14 page FAN1587A PRODUCT SPECIFICATION 8 REV. 1.0.5 11/10/03 Figure 12. Basic Regulator Circuit Load Regulation It is not possible to provide true remote load sensing because the FAN1587A series are three-terminal devices. Load regu- lation is limited by the resistance of the wire connecting the regulators 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 13. Figure 13. 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 regu- lation is obtained when the top of resistor divider R1 con- nects directly to the regulator output and not to the load. Figure 14 illustrates this point. If R1 connects to the load, then the effective resistance between the regulator and the load would be: RP × (1 + R2/R1), RP = Parasitic Line Resistance The connection shown in Figure 14 does not multiply RP by the divider ratio. 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 repre- sents 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 14. Connection for Best Load Regulation Thermal Considerations The FAN1587A series protect themselves under overload conditions 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-heat sink 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. For example, look at using an FAN1587AT to generate 3A @ 1.5V ± 2% from a 3.3V source (3.2V to 3.6V). Assumptions: •VIN = 3.6V worst case •VOUT = 1.46V worst case •IOUT = 3A continuous •TA = 70°C • θCase-to-Ambient = 3°C/W (assuming both a heatsink and a thermally conductive material) The power dissipation in this application is: PD = (VIN – VOUT) * (IOUT) = (3.6 – 1.46) * (3) = 6.42W From the specification table: TJ = TA + (PD) * (θCase-to-Ambient + θJC) = 70 + (6.42) * (3 + 3) = 109 °C The junction temperature is below the maximum rating. FAN1587A ADJ IADJ 35 µA R2 R1 C2 22 µF VOUT VREF + C1 22 µF + 65-1587-14 IN VOUT = VREF (1 + R2/R1) + IADJ (R2) OUT VIN FAN1587A-1.5, -3.3 GND RL 65-1587-17 RP PARASITIC LINE RESISTANCE IN OUT VIN FAN1587A ADJ R2* CONNECT R1 TO CASE CONNECT R2 TO LOAD * R1* RL 65-1587-15 RP PARASITIC LINE RESISTANCE IN OUT VIN |
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