LTC6906 datasheet(20/28 Pages) AD

20 / 28 page

LTC6995-1/LTC6995-2

Rev. B

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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 21. 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.Atapproximately500nA,theoscillatoroutput

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.

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.

APPLICATIONS INFORMATION

699512 F20

LTC6995-1

RST

GND

SET

OUT

V+

DIV

0.1µF

RSET

RVCO

VCTRL

V+

Figure 19.

Figure 20. Voltage-Controlled Oscillator

LTC6995-1

2.25V TO 5.5V

0.1µF

OUT

523k

RSET

178k

RST

GND

SET

OUT

V+

DIV

CPW

470pF

RPW

2.26k

699512 F19a

60 SECONDS

1µs PULSE WIDTH

VRST(RISING)

V+

tPULSE = –RPW • CPW • In

tPULSE ≈ –2.26k • 470pF • In(1 – 0.61)

tPULSE ≈ 1µs

1–

(

)

(a) Self-Resetting Circuit (DIVCODE = 5)

LTC6995-2

2.25V TO 5.5V

0.1µF

699512 F19b

OUT

523k

60 SECONDS

0.9µs PULSE WIDTH

VRST(FALLING)

V+

tPULSE = –RPW • CPW • In

tPULSE ≈ –2.26k • 470pF • In(0.43)

tPULSE ≈ 0.9µs

RSET

178k

RST

GND

SET

OUT

V+

DIV

CPW

470pF

RPW

2.26k

(

)

(b) Self-Resetting Circuit (DIVCODE = 10)

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