Datasheet
6
LTC1569-7
reduced. This results in a 60% power savings with a single
5V supply.
Table1. f
CUTOFF
vs R
EXT
, V
S
= 3V, T
A
= 25°C, Divide-by-1 Mode
R
EXT
Typical f
CUTOFF
Typical Variation of f
CUTOFF
3844Ω 320kHz ±3.0%
5010Ω 256kHz ±2.5%
10k 128kHz ±1%
20.18k 64kHz ±2.0%
40.2k 32kHz ±3.5%
The power reduction in the divide-by-4 and divide-by-16
modes, however, effects the fundamental oscillator fre-
quency. Hence, the effective divide ratio will be slightly
different from 4:1 or 16:1 depending on V
S
, T
A
and R
EXT
.
Typically this error is less than 1% (Figures 4 and 6).
Self-Clocking Operation
The LTC1569-7 features a unique internal oscillator which
sets the filter cutoff frequency using a single external
resistor
. The design is optimized for V
S
= 3V, f
CUTOFF
=
128kHz, where the filter cutoff frequency error is typically
<1% when a 0.1% external 10k resistor is used. With
different resistor values and internal divider settings, the
cutoff frequency can be accurately varied from 2kHz to
150kHz/300kHz (single 3V/5V supply). As shown in
Figure 1, the divider is controlled by the DIV/CLK (Pin 5).
Table 1 summarizes the cutoff frequency vs external
resistor values for the divide-by-1 mode.
In the divide-by-4 and divide-by-16 modes, the cutoff
frequencies in Table 1 will be lowered by 4 and 16
respectively. When the LTC1569-7 is in the divide-by-4
and divide-by-16 modes the power is automatically
APPLICATIONS INFORMATION
WUU
U
Figure 4. Typical Divide Ratio in the
Divide-by-4 Mode, T
A
= 25°C
Figure 3. Filter Cutoff vs Temperature,
Divide-by-1 Mode, R
EXT
= 10k
Figure 2. Filter Cutoff vs V
SUPPLY
,
Divide-by-1 Mode, T
A
= 25°C
Figure 1
V
SUPPLY
(V)
2
NORMALIZED FILTER CUTOFF
1569-7 F02
1.04
1.03
1.02
1.01
1.00
0.99
0.98
0.97
0.96
4 6 810
R
EXT
= 5k
R
EXT
= 10k
R
EXT
= 20k
R
EXT
= 40k
TEMPERATURE (°C)
–50
NORMALIZED FILTER CUTOFF
1569-7 F03
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 F04
4.08
4.04
4.00
3.96
4 6 810
R
EXT
= 5k
R
EXT
= 10k
R
EXT
= 20k
R
EXT
= 40k
LTC1569-7
18
27
36
45
DIVIDE-BY-4
DIVIDE-BY-1
DIVIDE-BY-16
V
+
V
–
R
EXT
100pF
f
CUTOFF
=
128kHz (10k/R
EXT
)
1, 4 OR 16
1569-7 F01
IN
+
IN
–
GND
V
–
OUT
V
+
R
X
DIV/CLK