LT1581 データシートの表示（PDF） - Linear Technology

部品番号

コンポーネント説明

メーカー

LT1581

10A, Very Low Dropout Regulators

Linear Technology 

LT1581/LT1581-2.5

APPLICATIONS INFORMATION

5V

CONTROL

3.3V

POWER

SENSE

LT1581

OUTPUT

ADJ

R1

R2

+

RP

LOAD VOUT

RP

–

1581 F01

Figure 1. Conventional Load Sensing

5V

CONTROL

3.3V

POWER

SENSE

LT1581

OUTPUT

ADJ

R1

R2

+

RP

LOAD VOUT

RP

–

1581 F02

Figure 2. Remote Load Sensing

VOUT

FIGURE 1

VOUT

FIGURE 2

IOUT

(∆IOUT)(RP)

TIME

1581 F03

Figure 3. Remote Sensing Improves Load Regulation

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

LT1581 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 LT1581 plus the voltage drop across RP.

Stability

The LT1581 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 LT1581 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 LT1581 is stable with the type of capacitors

recommended by processor manufacturers.

Bypassing the adjust terminal on the LT1581 improves

ripple rejection and transient response. The ADJUST 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

load current changes. Output capacitance can increase

without limit and larger values of output capacitance

further improve the stability and transient response of the

LT1581.

Modern microprocessors generate large high frequency

current transients. The load current step contains higher

order frequency components that the output coupling

7

Share Link: