LTC6900CS5 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6900CS5 Datasheet PDF : 12 Pages
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LTC6900
APPLICATIONS INFORMATION
3V OR 5V
S1
RSET1
RSET2
1 V+
5
OUT
( ) fOSC = 10MHz •
20k
N • RSET1
2 LTC6900
GND
V+
÷100
( ) OR
fOSC = 10MHz •
20k
N • RSET1//RSET2
3
4
SET DIV
÷10
6900 F07
÷1
Figure 7
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+
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 coefficient 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+
−1
⎡
( ) VRES
⎢
⎣⎢
fOSC(MAX )
fOSC(MIN)
⎤
− 1⎥
⎦⎥
(2)
V+
RSET
0.1μF
IPR
IRES
1 V+
5
OUT
2 LTC6900
GND
5V
÷100
3
4
SET DIV
÷10 OPEN
6900 F08
÷1
Figure 8. Concept for Programming via Current Steering
V+
+
VRES
RIN –
+
VIN
–
0.1μF
RSET
1 V+
5
OUT fOSC
LTC6900
2
GND
5V
÷100
3
4
SET DIV
÷10 OPEN
6900 F09
÷1
Figure 9. Implementation of Concept Shown in Figure 8
6900fa
9
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