Datasheet
7
LTC1569-7
APPLICATIONS INFORMATION
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Figure 5. Filter Cutoff vs Temperature,
Divide-by-4 Mode, R
EXT
= 10k
Figure 7. Filter Cutoff vs Temperature,
Divide-by-16 Mode, R
EXT
= 10k
The cutoff frequency is easily estimated from the equation
in Figure 1. Examples 1 and 2 illustrate how to use the
graphs in Figures 2 through 7 to get a more precise
estimate of the cutoff frequency.
Example 1: LTC1569-7, R
EXT
= 20k, V
S
= 3V, divide-by-16
mode, DIV/CLK (Pin␣ 5) connected to V
+
(Pin 7), T
A
= 25°C.
Using the equation in Figure 1, the approximate filter
cutoff frequency is f
CUTOFF
= 128kHz • (10k/20k)
• (1/16) = 4kHz.
For a more precise f
CUTOFF
estimate, use Table 1 to get
a value of f
CUTOFF
when R
EXT
= 20k and use the graph
in Figure 6 to find the correct divide ratio when V
S
= 3V
and R
EXT
= 20k. Based on Table 1 and Figure 6, f
CUTOFF
= 64kHz • (20.18k/20k) • (1/16.02) = 4.03kHz.
From Table 1, the part-to-part variation of f
CUTOFF
will
be ±2%. From the graph in Figure 7, the 0°C to 70°C
drift of f
CUTOFF
will be –0.2% to 0.2%.
Example 2: LTC1569-7, R
EXT
= 5k, V
S
= 5V, divide-by-1
mode, DIV/CLK (Pin␣ 5) connected to V
–
(Pin 4), T
A
= 25°C.
Using the equation in Figure 1, the approximate filter
cutoff frequency is f
CUTOFF
= 128kHz • (10k/5k)
• (1/1) = 256kHz.
For a more precise f
CUTOFF
estimate, use Table 1 to get
f
CUTOFF
frequency for R
EXT
= 5k and use Figure 2 to
correct for the supply voltage when V
S
= 5V. From
Table␣ 1 and Figure 2, f
CUTOFF
= 256k • (5.01k/5k) •
0.970 = 249kHz.
Figure 6. Typical Divide Ratio in the
Divide-by-16 Mode, T
A
= 25°C
TEMPERATURE (°C)
–50
NORMALIZED FILTER CUTOFF
1569-7 F05
1.010
1.008
1.006
1.004
1.002
1.000
0.998
0.996
0.994
0.992
0.990
–25
0 25 50 75 100
V
S
= 3V
V
S
= 5V
V
S
= 10V
V
SUPPLY
(V)
2
DIVIDE RATIO
1569-7 F06
16.32
16.16
16.00
15.84
4 6 810
R
EXT
= 5k
R
EXT
= 10k
R
EXT
= 20k
R
EXT
= 40k
TEMPERATURE (°C)
–50
NORMALIZED FILTER CUTOFF
1569-7 F07
1.010
1.008
1.006
1.004
1.002
1.000
0.998
0.996
0.994
0.992
0.990
–25
0 25 50 75 100
V
S
= 3V
V
S
= 5V
V
S
= 10V