Datasheet LT1581, LT1581-2.5 (Analog Devices) - 7

LT1581/LT1581-2.5
U U W U APPLICATIONS INFORMATION
5V LT1581 can control the voltage at the load as long as the input/output voltage is greater than the total of the dropout CONTROL voltage of the LT1581 plus the voltage drop across RP. 3.3V POWER SENSE LT1581
Stability
OUTPUT + The LT1581 requires the use of an output capacitor as part R ADJ P of the device frequency compensation. The device re- quires a minimum of 22µF tantalum or 150µF of aluminum LOAD VOUT R1 R2 electrolytic to ensure stability. Larger capacitor values RP – increase stability and improve transient performance. 1581 F01 Many different types of capacitors are available and have
Figure 1. Conventional Load Sensing
widely varying characteristics. These capacitors differ in capacitor tolerance (sometimes up to ±100%), equivalent 5V series resistance, equivalent series inductance and ca- pacitance temperature coefficient. The LT1581 frequency CONTROL compensation optimizes frequency response with low 3.3V POWER SENSE ESR capacitors. In general, use capacitors with an ESR of LT1581 less than 1Ω. OUTPUT + For microprocessor applications larger value capacitors R ADJ P will be needed to meet the transient requirements of the LOAD VOUT processor. Processor manufacturers require tight voltage R1 R2 tolerances on the power supply. High quality bypass RP – capacitors must be used to limit the high frequency noise 1581 F02 generated by the processor. Multiple small ceramic ca- pacitors in addition to high quality bulk tantalum capaci-
Figure 2. Remote Load Sensing
tors are typically required to limit parasitic inductance (ESL) and resistance (ESR) in the capacitors to acceptable (∆IOUT)(RP) levels. The LT1581 is stable with the type of capacitors recommended by processor manufacturers. VOUT FIGURE 1 Bypassing the adjust terminal on the LT1581 improves VOUT ripple rejection and transient response. The ADJUST pin is FIGURE 2 brought out on the fixed voltage device specifically to IOUT allow this capability. Capacitor values on the order of several hundred microfar- TIME 1581 F03 ads are used to ensure good transient response with heavy load current changes. Output capacitance can increase
Figure 3. Remote Sensing Improves Load Regulation
without limit and larger values of output capacitance further improve the stability and transient response of the illustrates the effect on output regulation. It is important to LT1581. note that the voltage drops due to RP are not eliminated. They will add to the dropout voltage of the regulator Modern microprocessors generate large high frequency regardless of whether they are inside the loop as in Figure current transients. The load current step contains higher 2 or outside the loop as in Figure 1. This means that the order frequency components that the output coupling 7