LTC6900CS5 Linear Technology, LTC6900CS5 Datasheet - Page 7

• Manufacturer Part Number: LTC6900CS5
• Manufacturer: Linear Technology
• Type: Silicon Oscillatorr

Specifications of LTC6900CS5

• Mounting Style: Surface Mount
• Screening Level: Industrial
• Product Length (mm): 2.9mm
• Product Depth (mm): 1.75mm
• Product Height (mm): 0.9mm
• Package / Case: TSOT-23
• Lead Free Status / RoHS Status: Not Compliant

APPLICATIO S I FOR ATIO
SELECTING THE DIVIDER SETTING AND RESISTOR
The LTC6900’s master oscillator has a frequency range
spanning 0.1MHz to 20MHz. 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
obtained using the lowest master oscillator frequency.
Lower master oscillator frequencies use less power and
are more accurate. For instance, f
obtained by either R
= 10MHz or R
The R
better accuracy.
Table 1. Frequency Range vs Divider Setting
DIVIDER SETTING
*
LTC6900 may experience 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.
Any resistor, R
oscillator, f
At master oscillator frequencies greater than 10MHz (R
1
10
100
(R
R
SET
SETMIN
SET
= 200k approach is preferred for lower power and
DIV (Pin 4) = GND
DIV (Pin 4) = Floating
DIV (Pin 4) = V
20
OSC
= 10k, R
SET
k
SET
.
•
= 200k, N = 10, master oscillator = 1MHz.
, tolerance adds to the inaccuracy of the
N f
10
U
•
SET
MHz
SETMAX
OSC
+
= 20k, N = 100, master oscillator
U
, N =
= 2M)
W
OSC
100
10
1
FREQUENCY RANGE
= 100kHz can be
50kHz to 1MHz
> 500kHz
OSC
< 100kHz
SET
< 20k ), the
) should be
U
*
ALTERNATIVE METHODS OF SETTING THE OUTPUT
FREQUENCY OF THE LTC6900
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
resistor R
already explained in the “Theory of Operation,” the voltage
difference between V
therefore, the Figure 3 circuit is less accurate than if a
resistor controls the oscillator frequency.
Figure 4 shows the LTC6900 configured as a VCO. A
voltage source is connected in series with an external 20k
resistor. The output frequency, f
V
V
relationship between the input current and the voltage
between V
degraded. The oscillator frequency, however, will mono-
tonically increase with decreasing V
CONTROL
+
1 A TO 100 A
and the SET pin. Again, this circuit decouples the
I
CONTROL
0V TO 1.1V
, that is the voltage source connected between
V
SET
CONTROL
+
Figure 4. Voltage Controlled Oscillator
Figure 3. Current Controlled Oscillator
V
is replaced by the ratio of 1.1V/I
and SET; the frequency accuracy will be
+
0.1 F
V
+
–
+
0.1 F
ƒ
I
R
CONTROL
20k
OSC
SET
+
TYPICAL f
ƒ
OSC
0.5%, V
8%, V
and SET is approximately 1.1V,
1
2
3
10MHz
EXPRESSED IN (A)
N
V
GND
SET
CONTROL
+
LTC6900
10MHz
CONTROL
1
2
3
OSC
N
•
182kHz TO 18MHz (TYPICALLY 8%)
V
GND
SET
20k
OUT
6900 F03
1.1V
LTC6900
ACCURACY
DIV
+
•
= 0.5V
R
20k
= 0V
SET
5
4
• I
OUT
6900 F04
DIV
OSC
CONTROL
CONTROL
• 1 –
N = 1
LTC6900
5
4
, will vary with
V
CONTROL
N = 1
1.1V
.
CONTROL
OSC
, the
7
. As
6900f