LTC6655CHMS8-2.5-TRPBF データシートの表示（PDF） - Linear Technology

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LTC6655CHMS8-2.5-TRPBF

0.25ppm Noise, Low Drift Precision Buffered Reference Family

Linear Technology 

Applications Information

The choice of output capacitor also affects the bandwidth

of the reference circuitry and resultant noise peaking. As

shown in Figure 1, the bandwidth is inversely proportional

to the value of the output capacitor.

Noise peaking is related to the phase margin of the output

buffer. Higher peaking generally indicates lower phase mar-

gin. Other factors affecting noise peaking are temperature,

input voltage, and output load current.

Start-Up and Load Transient Response

Results for the transient response plots (Figures 3 to 8)

were produced with the test circuit shown in Figure 2

unless otherwise indicated.

The turn-on time is slew limited and determined by the

short-circuit current, the output capacitor, and output

voltage as shown in the equation:

tON

=

VOUT

•

COUT

ISC

For example, the LTC6655-2.5V, with a 3.3µF output

capacitor and a typical short-circuit current of 20mA, the

start-up time would be approximately:

2.5V

•

3.3 • 10–6F

0.02A

=

412µs

The resulting turn-on time is shown in Figure 3. Here

the output capacitor is 3.3µF and the input capacitor is

0.1µF.

Figure 4 shows the output response to a 500mV step on

VIN. The output response to a current step sourcing and

sinking is shown in Figures 5 and 6, respectively.

Figure 7 shows the output response as the current goes

from sourcing to sinking.

Shutdown Mode

The LTC6655 family of references can be shut down by

tying the SHDN pin to ground. There is an internal pull-up

resistor tied to this pin. If left unconnected this pin rises to

VIN and the part is enabled. Due to the low internal pull-up

current, it is recommended that the SHDN pin be pulled

high externally for normal operation to prevent accidental

LTC6655

120

COUT = 2.7µF

100

80

COUT = 10µF

60

COUT = 100µF

40

20

0

0.01

0.1

1

10

100

FREQUENCY (kHz)

1000

6655 F01

Figure 1. Output Voltage Noise Spectrum

7 VOUT 100Ω

VIN

3V

1,2

LTC6655-2.5

6

CIN

0.1µF

3,4,5,8

COUT

3.3µF

VGEN

6655 F02

0.5V

Figure 2. Transient Load Test Circuit

VIN

2V/DIV

VOUT

1V/DIV

COUT = 3.3µF

200µs/DIV

6655 F03

Figure 3. Start-Up Response

3.5V

VIN

3V

VOUT

50mV/DIV

COUT = 3.3µF

400µs/DIV

6655 F04

Figure 4. Output Response with a 500mV Step On VIN

6655fa

11

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