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LT1580IR-2.5 Datasheet(PDF) 8 Page - Linear Technology |
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LT1580IR-2.5 Datasheet(HTML) 8 Page - Linear Technology |
8 / 16 page 8 LT1580/LT1580-2.5 APPLICATIONS INFORMATION values of RP. Trace B of Figure 3 illustrates the effect on output regulation. It is important to note that the voltage drops due to RP are not eliminated. They will add to the dropout voltage of the regulator regardless of whether they are inside the loop as in Figure 2 or outside the loop as in Figure 1. This means that the LT1580 can control the voltage at the load as long as the input-output voltage is greater than the total of the dropout voltage of the LT1580 plus the voltage drop across RP. Stability The LT1580 requires the use of an output capacitor as part of the device frequency compensation. The device re- quires a minimum of 22 µFtantalumor150µFofaluminum electrolytic to ensure stability. Larger capacitor values increase stability and improve transient performance. Many different types of capacitors are available and have widely varying characteristics. These capacitors differ in capacitor tolerance (sometimes up to ±100%), equivalent series resistance, equivalent series inductance and ca- pacitance temperature coefficient. The LT1580 frequency compensation optimizes frequency response with low ESR capacitors. In general, use capacitors with an ESR of less than 1 Ω. For microprocessor applications larger value capacitors will be needed to meet the transient requirements of the processor. Processor manufacturers require tight voltage tolerances on the power supply. High quality bypass capacitors must be used to limit the high frequency noise generated by the processor. Multiple small ceramic ca- pacitors in addition to high quality bulk tantalum capaci- tors are typically required to limit parasitic inductance (ESL) and resistance (ESR) in the capacitors to acceptable levels. The LT1580 is stable with the type of capacitors recommended by processor manufacturers. Bypassing the adjust terminal on the LT1580 improves ripple rejection and transient response. The ADJ pin is brought out on the fixed voltage device specifically to allow this capability. Capacitor values on the order of several hundred microfar- ads are used to ensure good transient response with heavy pin and the top of the resistor divider are connected to the top of the load. The bottom of the resistor divider is connected to the bottom of the load. RP is now effectively connected inside the regulating loop of the LT1580 and the load regulation at the load will be negligible for reasonable VOUT SENSE ADJ R2 R1 1580 F02 RP RP LT1580 VPOWER 3.3V 5V VCONTROL LOAD VOUT + – VOUT SENSE ADJ R2 R1 1580 F01 RP RP LT1580 VPOWER 3.3V 5V VCONTROL LOAD VOUT + – Figure 1. Conventional Load Sensing Figure 3. Remote Sensing Improves Load Regulation VOUT FIGURE 1 ( ∆IOUT)(RP) TIME 1580 F03 VOUT FIGURE 2 IOUT Figure 2. Remote Load Sensing |
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