LT6658 データシートの表示（PDF） - Linear Technology

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LT6658

Precision Dual Output, High Current, Low Noise, Voltage Reference

Linear Technology 

LT6658

Applications Information

LT6658-2.5

VPAR

+–

+

VOUT2_F 12 RPAR

ILOAD

–

VOUT2_S 13

RLOAD

a)

LT6658-2.5

VPAR

+–

+

VOUT2_F 12 RPAR

–

VOUT2_S 13

ILOAD

RLOAD

6658 F12

b)

Figure 12. How to Make a Proper Kelvin Sense Connection

Output Noise and Noise Reduction (NR)

The LT6658 noise characteristic is similar to that of a high

performance reference. The total noise is a combination

of the bandgap noise and the noise of the buffer amplifier.

The bandgap noise can be measured at the NR pin and

is shown in Figure 13 with a 1μF capacitor, 10µF capaci-

tor and no capacitor on the NR pin. The bandgap can be

bandlimited by connecting a capacitor between the NR

pin and ground. The RC product sets the low pass 3dB

corner attenuating the out-of-band noise of the bandgap.

An internal 400Ω ±15% resistor combines with the external

capacitor to create a single-pole low pass filter. Table 1

lists capacitor values and the corresponding 3dB cutoff

frequency.

250

CNR = 0µF

200

150

Table 1. NR Capacitor Values and the Corresponding 3dB Frequency

NR Capacitor (µF)

NR 3dB Frequency (Hz)

0.1

3979

0.22

1809

0.47

847

1

398

2.2

181

4.7

85

10

40

22

18

The primary trade-off for including an RC filter on the

NR pin is a slower turn-on time. The effective resistance

seen by the NR capacitor is 400Ω. The RC time constant

(τ) for charging the NR capacitor is τ = R • C. To reach

the initial accuracy specification for the LT6658, 0.05%,

it will take 7.6τ of settling time. Example settling time

constants are shown in Table 2. An example of the NR

pin charging and the relationship to the output voltage

is shown in Figure 14. The appropriate trade-off between

settling time and noise limiting is specific to the demands

of each unique application.

Table 2. Settling Times for Different NR Capacitor Values

Output Voltage NR Pin Resistance

C

7.6τ

(V)

(Ω)

(μF)

(ms)

2.5

400

0.01

0.030

0.1

0.30

1

3.04

5V/DIV VIN

1V/DIV VNR

100

CNR = 1µF

50

CNR = 10µF

0

0.01 0.1

1

10

FREQUENCY (kHz)

100 1000

6658 F13

Figure 13. LT6658 Bandgap Output Voltage Noise

1V/DIV VOUT1

500µs/DIV

CNR = 1µF

COUT1 = 1µF

6658 F14

Figure 14. Start-up Response on the NR pin and VOUT_F

For more information www.linear.com/LT6658

6658f

17

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