Datasheet > AB-099

AB-099

更新时间： 2024-11-22 23:26:55

品牌	Logo	应用领域
其他 - ETC		运算放大器
页数	文件大小	规格书
2页	29K
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AB-099 - AN EASY SOLUTION TO CURRENT LIMITING AN OP AMP

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AB-099 数据手册

AP P LICATION BULLETIN

Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706

Tel: (520) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (520) 889-1510 • Immediate Product Info: (800) 548-6132

AN EASY SOLUTION TO CURRENT

LIMITING AN OP AMP

By Richard Kulavik

Many circuits today not only require voltage regulation but

Figure 1 shows two REG1117s (U₁, U₂) with R₁, from the

V_OUTpins to the Adj pins allowing

they also need current regulation. Some systems draw ex-

cessive current during power on, which can cause expensive

system components to burn up due to improper powering of

supplies. This is not the only problem associated with

excessive currents. Some applications need to limit the

current an op amp can provide to a specific load. This circuit

can provide this type of function by making constant current

sources out of simple regulators.

I_REG(MAX)= (1.25V/R₁) – 50µA

to be sources from the floating output to the system. If the

system does not sink or source more than or equal to I_REG(MAX)

the regulators will be out of regulation and will drop its

voltage across V_OUT, to Adj equalling 1.25V + I_REG• R₁,

where I_REGis equal to the current demands of the system.

When the current demands of the system rise and approach

1.25V/R₁, the regulator will begin to maintain its regulation

and provide a maximum current, limited by R₁. At this point,

the maximum voltage drop will be equal to 1.2V + 1.25V or

2.45V from V_INto Adj. The supply voltage minus 2.45V will

determine the compliance voltage of the current regulator.

The REG1117 can source from 10mA to 800mA using the

circuits shown.

The REG1117 is a low cost, low drop out, adjustable

regulator that can be used for both voltage and current

regulation. In the normal mode of operation, applying a

fixed voltage on the input pin will provide a fixed voltage

between V_OUTand Adj. Placing a resistor across V_OUTand

Adj will limit the amount of current delivered to the load by

making the regulator appear as a constant current source.

Two problems can arise from creating a circuit such as the

one described in Figure 1. The first problem is that the

REG1117s have a transient response that will affect the

output of the amplifier. Figure 2 shows the output voltage

deviation in millivolts versus time from the REG1117. This

voltage transient will appear as a ringing voltage to the load

of the amplifier. If bypass capacitors are added on the power

supply pins of an amplifier (A₁), the ringing can be reduced

at the cost of greater overshoot and longer settling times.

Remember that bypass capacitors are on the supply leads of

+V_S

V_OUT

I_REG

REG1117

R₁

Adj

50µA

LINE TRANSIENT RESPONSE

C_BYPASS

C_IN= 1µF

C_OUT= 10µF Tantalum

System

Load

I_OUT= 0.1A

–20

V_OUT

C_BYPASS

REG1117

Adj

I_REG

–40

R₁

5.25

4.25

3.25

–V_S

80 100 120 140 160 180 200

Time (µs)

FIGURE 2. Transient Response of the REG1117.

FIGURE 1. A Voltage Regulator Can Be Used as a Current

Regulator and an Op Amp.

AB-099

Printed in U.S.A. May, 1995

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