6

LTC1569-7

reduced. This results in a 60% power savings with a single

5V supply.

Table1. fCUTOFF vs REXT, VS = 3V, TA = 25°C, Divide-by-1 Mode

REXT

Typical fCUTOFF

Typical Variation of fCUTOFF

3844

Ω

320kHz

±3.0%

5010

Ω

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 VS, TA and REXT.

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 VS = 3V, fCUTOFF =

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

Figure 4. Typical Divide Ratio in the

Divide-by-4 Mode, TA = 25°C

Figure 3. Filter Cutoff vs Temperature,

Divide-by-1 Mode, REXT = 10k

Figure 2. Filter Cutoff vs VSUPPLY,

Divide-by-1 Mode, TA = 25°C

Figure 1

VSUPPLY (V)

2

1569-7 F02

1.04

1.03

1.02

1.01

1.00

0.99

0.98

0.97

0.96

4

6

810

REXT = 5k

REXT = 10k

REXT = 20k

REXT = 40k

TEMPERATURE (

°C)

–50

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

VS = 3V

VS = 5V

VS = 10V

VSUPPLY (V)

2

1569-7 F04

4.08

4.04

4.00

3.96

4

6

810

REXT = 5k

REXT = 10k

REXT = 20k

REXT = 40k

LTC1569-7

18

27

36

45

DIVIDE-BY-4

DIVIDE-BY-1

DIVIDE-BY-16

V+

V–

REXT

100pF

fCUTOFF =

128kHz (10k/REXT)

1, 4 OR 16

1569-7 F01

IN+

IN–

GND

V–

OUT

V+

RX

DIV/CLK