7

LTC1569-7

APPLICATIONS INFORMATION

Figure 5. Filter Cutoff vs Temperature,

Divide-by-4 Mode, REXT = 10k

Figure 7. Filter Cutoff vs Temperature,

Divide-by-16 Mode, REXT = 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, REXT = 20k, VS = 3V, divide-by-16

mode, DIV/CLK (Pin 5) connected to V + (Pin 7), TA = 25°C.

Using the equation in Figure 1, the approximate filter

cutoff frequency is fCUTOFF = 128kHz • (10k/20k)

• (1/16) = 4kHz.

For a more precise fCUTOFF estimate, use Table 1 to get

a value of fCUTOFF when REXT = 20k and use the graph

in Figure 6 to find the correct divide ratio when VS = 3V

and REXT = 20k. Based on Table 1 and Figure 6, fCUTOFF

= 64kHz • (20.18k/20k) • (1/16.02) = 4.03kHz.

From Table 1, the part-to-part variation of fCUTOFF will

be

±2%. From the graph in Figure 7, the 0°C to 70°C

drift of fCUTOFF will be – 0.2% to 0.2%.

Example 2: LTC1569-7, REXT = 5k, VS = 5V, divide-by-1

mode, DIV/CLK (Pin 5) connected to V – (Pin 4), TA = 25°C.

Using the equation in Figure 1, the approximate filter

cutoff frequency is fCUTOFF = 128kHz • (10k/5k)

• (1/1) = 256kHz.

For a more precise fCUTOFF estimate, use Table 1 to get

fCUTOFF frequency for REXT = 5k and use Figure 2 to

correct for the supply voltage when VS = 5V. From

Table 1 and Figure 2, fCUTOFF = 256k • (5.01k/5k) •

0.970 = 249kHz.

Figure 6. Typical Divide Ratio in the

Divide-by-16 Mode, TA = 25°C

TEMPERATURE (

°C)

–50

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

VS = 3V

VS = 5V

VS = 10V

VSUPPLY (V)

2

1569-7 F06

16.32

16.16

16.00

15.84

4

6

810

REXT = 5k

REXT = 10k

REXT = 20k

REXT = 40k

TEMPERATURE (

°C)

–50

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

VS = 3V

VS = 5V

VS = 10V