LTC1799
APPLICATIO S I FOR ATIO
SELECTING THE DIVIDER SETTING AND RESISTOR
The LTC1799’s master oscillator has a frequency range
spanning 0.1MHz to 33MHz. However, accuracy may
suffer if the master oscillator is operated at greater than
10MHz with a supply voltage lower than 4V. A program-
mable divider extends the frequency range to greater than
three decades. Table 1 describes the recommended fre-
quencies for each divider setting. Note that the ranges
overlap; at some frequencies there are two divider/resistor
combinations that result in the desired frequency.
In general, any given oscillator frequency (f
OSC
) should be
obtained using the lowest master oscillator frequency.
Lower master oscillator frequencies use less power and
are more accurate. For instance, f
OSC
= 100kHz can be
obtained by either R
SET
= 10k, N = 100, master oscillator
= 10MHz or R
SET
= 100k, N = 10, master oscillator = 1MHz.
The R
SET
= 100k is preferred for lower power and better
accuracy.
Table 1. Frequency Range vs Divider Setting
DIVIDER SETTING
÷1
÷10
⇒
⇒
DIV (Pin 4) = GND
DIV (Pin 4) = Floating
DIV (Pin 4) = V
+
FREQUENCY RANGE
> 500kHz
*
50kHz to 1MHz
< 100kHz
÷100 ⇒
*
At master oscillator frequencies greater than 10MHz (R
SET
< 10kΩ), the
LTC1799 may suffer reduced accuracy with a supply voltage less than 4V.
After choosing the proper divider setting, determine the
correct frequency-setting resistor. Because of the linear
correspondence between oscillation period and resis-
tance, a simple equation relates resistance with frequency.
100
10MHz
R
SET
=
10k •
, N =
10
N • f
OSC
1
(R
SETMIN
= 3k (5V Supply), 5k (3V Supply),
R
SETMAX
= 1M)
Any resistor, R
SET
, tolerance adds to the inaccuracy of the
oscillator, f
OSC
.
U
ALTERNATIVE METHODS OF SETTING THE OUTPUT
FREQUENCY OF THE LTC1799
The oscillator may be programmed by any method that
sources a current into the SET pin (Pin 3). The circuit in
Figure 3 sets the oscillator frequency using a program-
mable current source and in the expression for f
OSC
, the
resistor R
SET
is replaced by the ratio of 1.13V/I
CONTROL
. As
already explained in the “Theory of Operation,” the voltage
difference between V
+
and SET is approximately 1.13V,
therefore, the Figure 3 circuit is less accurate than if a
resistor controls the oscillator frequency.
Figure 4 shows the LTC1799 configured as a VCO. A
voltage source is connected in series with an external 10k
resistor. The output frequency, f
OSC
, will vary with
V
CONTROL
, that is the voltage source connected between
V
+
and the SET pin. Again, this circuit decouples the
relationship between the input current and the voltage
between V
+
and SET; the frequency accuracy will be
degraded. The oscillator frequency, however, will mono-
tonically increase with decreasing V
CONTROL
.
V
+
0.1µF
400kHz TO 21MHz
(APPROXIMATE, SEE TEXT)
1
2
3
OUT
V
+
LTC1799
GND
SET
DIV
1799 F03
W
U U
5
I
CONTROL
5µA TO 200µA
4
N=1
ƒ
OSC
≅
10MHz • 10kΩ • I
CONTROL
N
1.13V
I
CONTROL
EXPRESSED IN (A)
Figure 3. Current Controlled Oscillator
V
+
0.1µF
R
SET
10k
1
2
3
V
CONTROL
0V TO 1.13V
+
–
V
+
OUT
LTC1799
GND
SET
DIV
1799 F04
5
4
N=1
ƒ
OSC
≅
10MHz • 10k • 1 – V
CONTROL
N
R
SET
1.13V
(
)
Figure 4. Voltage Controlled Oscillator
7
LINEAR [ LINEAR INTEGRATED SYSTEMS ]
