LTC6907 datasheet(17/24 Pages) AD

17 / 24 page

LTC6991

Rev. D

For more information www.analog.com

Digital Frequency Control

The control voltage can be generated by a DAC (digital-

to-analog converter), resulting in a digitally-controlled

frequency. Many DACs allow for the use of an external

reference. If such a DAC is used to provide the VCTRL

voltage, the VSET dependency can be eliminated by buffer-

ing VSET and using it as the DAC’s reference voltage, as

shown in Figure 15. The DAC’s output voltage now tracks

any VSET variation and eliminates it as an error source.

The SET pin cannot be tied directly to the reference input

of the DAC because the current drawn by the DAC’s REF

input would affect the frequency.

ISET Extremes (Master Oscillator Frequency Extremes)

When operating with ISET outside of the recommended

1.25µA to 20µA range, the master oscillator operates

outside of the 62.5kHz to 1MHz range in which it is most

accurate.

The oscillator can still function with reduced accuracy

for ISET < 1.25µA. At approximately 500nA, the oscillator

output will be frozen in its current state. The output could

halt in a high or low state. This avoids introducing short

pulses when frequency modulating a very low frequency

output.

APPLICATIONS INFORMATION

–

6991 F15

LTC6991

RST

GND

SET

OUT

V+

DIV

0.1µF

RSET

V+

RVCO

V+

0.1µF

1/2

LTC6078

LTC1659

V+

VCC

REF

GND

VOUT

µP

DIN

CLK

CS/LD

1MHz • 50k

1024 • NDIV • RVCO

fOUT =

DIN = 0 TO 4095

• 1 +

–

RVCO

RSET

DIN

4096

(

)

0.1µF

Figure 15. Digitally-Controlled Oscillator

At the other extreme, it is not recommended to operate

the master oscillator beyond 2MHz because the accuracy

of the DIV pin ADC will suffer.

Frequency Modulation and Settling Time

The LTC6991 will respond to changes in ISET up to a –3dB

bandwidth of 0.4 • fOUT.

Following a 2× or 0.5× step change in ISET, the output

frequency takes less than one cycle to settle to within 1%

of the final value.

Power Supply Current

The power supply current varies with frequency, supply

voltage and output loading. It can be estimated under

any condition using the following equation. This equation

ignores CLOAD (valid for CLOAD < 1nF) and assumes the

output has 50% duty cycle.

IS(TYP) ≈ V+ • fMASTER • 7.8pF +

V+

420kΩ

V+

2 • RLOAD

+ 1.8 • ISET + 50µA

LTC6907 Datasheet(PDF) 17 Page - Analog Devices

LTC6907 Datasheet(HTML) 17 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Datasheet Download

Link URL