6

LTC1569-6

BLOCK DIAGRA

10TH ORDER

LINEAR PHASE

FILTER NETWORK

POWER

CONTROL

DIVIDER/

BUFFER

REXT

PRECISION

OSCILLATOR

5

6

7

8

4

3

2

1

OUT

V+

RX

DIV/CLK

IN+

IN–

GND

V–

1569-6 BD

APPLICATIONS INFORMATION

Self-Clocking Operation

The LTC1569-6 features a unique internal oscillator which

sets the filter cutoff frequency using a single external

resistor. The design is optimized for VS = 3V, fCUTOFF =

64kHz, 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 1kHz to

64kHz. 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.

LTC1569-6

18

27

36

45

DIVIDE-BY-4

DIVIDE-BY-1

DIVIDE-BY-16

V+

V–

REXT

100pF

fCUTOFF =

64kHz (10k/REXT)

1, 4 OR 16

1569-6 F01

IN+

IN–

GND

V–

OUT

V+

RX

DIV/CLK

Figure 1

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

REXT

Typical fCUTOFF

Typical Variation of fCUTOFF

3844

Ω*

N/A

±3.0%

5010

Ω*

N/A

±2.5%

10k

64kHz

±1%

20.18k

32kHz

±2.0%

40.2k

16kHz

±3.5%

*REXT values less than 10k can be used only in the divide-by-16 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 LTC1569-6 is in the divide-by-4

and divide-by-16 modes the power is automatically re-

duced. This results in up to a 40% power savings.

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).

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: LTC1569-6, REXT = 20k, VS = 3V, divide-by-16

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