LT2178 datasheet(17/28 Pages) LINER

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LTC6258/LTC6259/LTC6260

6258960fa

For more information www.linear.com/LTC6258

APPLICATIONS INFORMATION

–

V+

OUT

LTC6258

6258960 F05a

22µF

RIN1

2.7k

22µF

OUT

GND

LT6656

Figure 5a. Low Noise Reference Use LT6656 for a Low Current Starting Reference

Figure 5b. Noise Density, Reference Buffer

Figure 5c. Reference Buffer Transient Response

* 2.7k + 22µF FILTER

OP AMP, 44µF CLOAD

OP AMP

FILTERED REFERENCE

REFERENCE OUTPUT

INSTRUMENT ONLY

FREQUENCY (kHz)

0.01

0.1

100

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

6258 F05b

This circuit takes advantage of the ability of the LTC6259 to

drivelargecapacitiveloads.Useofalargeoutputcapacitor

attached to the LT6659 enables significant bypassing of

follow-oncircuitsthatusethereferencevoltage.Intotal,the

combinationofLT6656andLTC6259,inthisconfiguration,

develops a reference voltage, with low noise, at low power,

and with appreciably large available bypass capacitance.

Voltage spectral noise densities are shown in Figure 5b.

Larger noise from the reference below 10kHz noticeably

drops down once a filter (RIN1 and CIN1) follow the refer-

ence. The op amp, configured in unity gain, with a large

44µF load, remains stable and contributes only a small

amount of low frequency noise. Figure 5c shows the tran-

sient response of the combination of RIN1-CIN1 filter and

op amp circuit, with and without the 44µF output capacitor.

Figure 5c shows the time domain response of the refer-

ence buffer.

The total measured supply current consumption is 21µA.

Analog LED Control

Figure 6a shows a voltage controlled LED drive circuit.

When VIN is at 0V, the op amp supply current is nominally

20μA. The offset, for example, could be 450µV, appears

across R1, inducing a 0.45mA current in the LED. Some

applications want a guaranteed zero LED current at

VIN = 0, and this is the purpose of R5. R5 forces 2.5μA

currentthroughR7,creatinganegative0.6mVsenseoffset.

This offset guarantees a zero LED current.

INPUT

NO 44µF OUTPUT CAP

ADD 44µF OUTPUT CAP

200ms/Div

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

6258 F05c

LT2178 데이터시트(PDF) 17 Page - Linear Technology

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