Datasheet

LTC6995-1/LTC6995-2
20
699512fa
For more information www.linear.com/LTC6995-1
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 V
CTRL
voltage, the V
SET
dependency can be eliminated by buffer-
ing V
SET
and using it as the DAC’s reference voltage, as
shown in Figure 21. The DAC’s output voltage now tracks
any V
SET
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.
I
SET
Extremes (Master Oscillator Frequency Extremes)
When operating with I
SET
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
I
SET
< 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.
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
LTC6995-1
2.25V TO 5.5V
0.1µF
OUT
R1
1M
R2
523k
R
SET
178k
RST
GND
SET
OUT
V
+
DIV
C
PW
470pF
R
PW
2.26k
699512 F19a
60 SECONDS
1µs PULSE WIDTH
V
RST(RISING)
V
+
t
PULSE
= –R
PW
• C
PW
• In
t
PULSE
≈ –2.26kΩ • 470pF • In(1 – 0.61)
t
PULSE
≈ 1µs
1–
( )
LTC6995-2
2.25V TO 5.5V
0.1µF
699512 F19b
OUT
R1
523k
R2
1M
60 SECONDS
0.9µs PULSE WIDTH
V
RST(FALLING)
V
+
t
PULSE
= –R
PW
• C
PW
• In
t
PULSE
≈ –2.26kΩ • 470pF • In(0.43)
t
PULSE
≈ 0.9µs
R
SET
178k
RST
GND
SET
OUT
V
+
DIV
C
PW
470pF
R
PW
2.26k
( )
699512 F20
LTC6995-1
RST
GND
SET
OUT
V
+
DIV
R1
C1
0.1µF
R2
R
SET
R
VCO
V
CTRL
V
+
Figure 19a. Self-Resetting Circuit (DIVCODE = 5)
Figure 19b. Self-Resetting Circuit (DIVCODE = 10)
Figure 20. Voltage-Controlled Oscillator