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LT1585CM Datasheet(PDF) 10 Page - Linear Technology |
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LT1585CM Datasheet(HTML) 10 Page - Linear Technology |
10 / 12 page 10 LT1584/LT1585/LT1587 APPLICATIONS INFORMATION load current of 10mA. The current out of the adjust pin adds to the current from R1 and is typically 55 µA. Its output voltage contribution is small and only needs consideration when very precise output voltage setting is required. LT1584 ADJ VIN VOUT IN R1 VREF R2 VOUT = VREF (1 + R2/R1) + IADJ (R2) OUT IADJ 55 µA LT1585 • F03 C1 10 µF + C2 22 µF + Figure 3. Basic Adjustable Regulator Load Regulation It is not possible to provide true remote load sensing because the LT1584/LT1585/LT1587 are three-terminal devices. Load regulation 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 re- turned to the negative side of the load. This is illustrated in Figure 4. LT1584-3.3 RP PARASITIC LINE RESISTANCE GND VIN RL IN OUT LT1585 • F04 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 regulation is obtained when the top of 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, the effective resistance between the regulator and the load is: RP × (1 + R2/R1), RP = Parasitic Line Resistance The connection shown in Figure 5 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 represents a significant percentage of the overall regula- tion. 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 LT1584 OUT IN VIN ADJ RP PARASITIC LINE RESISTANCE R1* *CONNECT R1 TO CASE CONNECT R2 TO LOAD LT1584 • F05 RL R2* Thermal Considerations The LT1584/LT1585/LT1587 family protects the device 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 important to consider all sources of thermal resistance from junction-to-ambient. These sources in- clude 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 temperature and ensure safe operating temperatures. The electrical characteristics sec- tion provides a separate thermal resistance and maximum junction temperature for both the control circuitry and the power transistor. Older regulators, with a single junction- to-case thermal resistance specification, use an average of the two values provided here and allow excessive junction temperatures under certain conditions of ambient tem- perature and heat sink resistance. Calculate the maximum junction temperature for both sections to ensure that both thermal limits are met. Junction-to-case thermal resistance is specified from the IC junction to the bottom of the case directly below the die. |
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