LT1580 데이터 시트보기 (PDF) - Linear Technology

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LT1580

7A, Very Low Dropout Regulator

Linear Technology 

LT1580/LT1580 -2.5

APPLICATIONS INFORMATION

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

5V

VCONTROL

3.3V

VPOWER

SENSE

LT1580

VOUT

+

ADJ

RP

R1

R2

LOAD VOUT

RP

–

1580 F01

Figure 1. Conventional Load Sensing

5V

VCONTROL

3.3V

VPOWER

SENSE

LT1580

VOUT

ADJ

R1

R2

+

RP

LOAD VOUT

RP

–

1580 F02

Figure 2. Remote Load Sensing

VOUT

FIGURE 1

VOUT

FIGURE 2

IOUT

(∆IOUT)(RP)

TIME

1580 F03

Figure 3. Remote Sensing Improves Load Regulation

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 22µF tantalum or 150µF of aluminum

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 1Ω.

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

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