Datasheet

LTC6991
16
6991fb
Self-Resetting Circuits
The RST pin has hysteresis to accommodate slow-changing
input voltages. Furthermore, the trip points are proportional
to the supply voltage (see Note 6 and the RST Threshold
Voltage vs Supply Voltage curve in Typical Performance
Characteristics). This allows an RC time constant at the
RST input to generate a delay that is nearly independent
of the supply voltage.
A simple application of this technique allows the LTC6991
output to reset itself, producing a well-controlled pulse
once each cycle. Figures 13a and 13b show circuits that
produce approximately 1µs pulses once a minute. The
only difference is in the POL bit setting, which controls
whether the pulse is positive or negative.
Voltage Controlled Frequency
With one additional resistor, the LTC6991 output frequency
can be manipulated by an external voltage. As shown in
Figure 14, voltage V
CTRL
sources/sinks a current through
R
VCO
to vary the I
SET
current, which in turn modulates the
output frequency as described in Equation (3).
f
MHz k
NR
R
R
V
V
OUT
DIV VCO
VCO
SET
CTRL
S
=+
150
1024
1
s
ss
sn
Ω
EET
⎛
⎝
⎜
⎞
⎠
⎟
(3)
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 15. 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.
APPLICATIONS INFORMATION
LTC6991
2.25V TO 5.5V
0.1µF
OUT
R1
1M
R2
392k
R
SET
715k
RST
GND
SET
OUT
V
+
DIV
C
PW
470pF
R
PW
2.26k
6991 F13a
60 SECONDS
1µs PULSE WIDTH
V
RST(RISING)
V
+
t
PULSE
= –R
PW
• C
PW
• In
t
PULSE
≈ –2.26k • 470pF • In(1 – 0.61)
t
PULSE
≈ 1µs
1–

LTC6991
2.25V TO 5.5V
0.1µF
6991 F13b
OUT
R1
392k
R2
1M
60 SECONDS
0.9µs PULSE WIDTH
V
RST(FALLING)
V
+
t
PULSE
= –R
PW
• C
PW
• In
t
PULSE
≈ –2.26k • 470pF • In(0.43)
t
PULSE
≈ 0.9µs
R
SET
715k
RST
GND
SET
OUT
V
+
DIV
C
PW
470pF
R
PW
2.26k

6991 F14
LTC6991
RST
GND
SET
OUT
V
+
DIV
R1
C1
0.1µF
R2
R
SET
R
VCO
V
CTRL
V
+
Figure 13a. Self-Resetting Circuit (DIVCODE = 4)
Figure 13b. Self-Resetting Circuit (DIVCODE = 11)
Figure 14. Voltage-Controlled Oscillator