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LTC6900
9
6900fa
APPLICATIONS INFORMATION
A Ground Referenced Voltage Controlled Oscillator
The LTC6900 output frequency can also be programmed by
steering current in or out of the SET pin, as conceptually
shown in Figure 8. This technique can degrade accuracy
as the ratio of (V+ – VSET) / IRES is no longer uniquely
dependent of the value of RSET, as shown in the LTC6900
Block Diagram. This loss of accuracy will become noticeable
when the magnitude of IPROG is comparable to IRES. The
frequency variation of the LTC6900 is still monotonic.
Figure 9 shows how to implement the concept shown in
Figure 8 by connecting a second resistor, RIN, between the
SET pin and a ground referenced voltage source, VIN.
For a given power supply voltage in Figure 9, the output
frequency of the LTC6900 is a function of VIN, RIN, RSET
and (V+ – VSET) = VRES:
fOSC =
10MHz
N
20k
RIN RSET
1
+
VIN V+
()
VRES
1
+
RIN
RSET
(1)
When VIN = V+, the output frequency of the LTC6900 as-
sumes the highest value and it is set by the parallel com-
bination of RIN and RSET. Also note, the output frequency,
fOSC, is independent of the value of VRES = (V+ – VSET) so
the accuracy of fOSC is within the data sheet limits.
When VIN is less than V+, and expecially when VIN ap-
proaches the ground potential, the oscillator frequency,
fOSC, assumes its lowest value and its accuracy is affected
by the change of VRES = (V+ – VSET). At 25°C VRES varies
by ±8%, assuming the variation of V+ is ±5%. The tem-
perature coefcient of VRES is 0.02%/°C.
By manipulating the algebraic relation for fOSC above, a
simple algorithm can be derived to set the values of external
resistors RSET and RIN, as shown in Figure 9.
1. Choose the desired value of the maximum oscillator
frequency, fOSC(MAX), occurring at maximum input
voltage VIN(MAX) ≤ V+.
2. Set the desired value of the minimum oscillator fre-
quency, fOSC(MIN), occurring at minimum input voltage
VIN(MIN) ≥ 0.
3. Choose VRES = 1.1 and calculate the ratio of RIN/RSET
from the following:
RIN
RSET
=
VIN(MAX) V+
() fOSC(MAX)
fOSC(MIN)
VIN(MIN) V
+
()
VRES
fOSC(MAX)
()
fOSC(MIN)
1
1
(2)
Figure 7
V+
1
2
RSET1
RSET2
3
S1
5
V+
6900 F07
4
GND
LTC6900
3V OR 5V
SET
OUT
DIV
÷10
÷100
÷1
fOSC = 10MHz
OR
()
20k
N RSET1
fOSC = 10MHz
()
20k
N RSET1//RSET2
Figure 8. Concept for Programming via Current Steering
Figure 9. Implementation of Concept Shown in Figure 8
V+
1
2
RSET
IPR
3
5
5V
V+
6900 F08
4
GND
LTC6900
0.1μF
OPEN
SET
OUT
DIV
÷10
÷100
÷1
IRES
V+
1
2
RSET
VRES
RIN
VIN
3
5
5V
V+
6900 F09
4
GND
LTC6900
0.1μF
fOSC
OPEN
SET
OUT
DIV
÷10
÷100
÷1
+
–
+
–
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