LTC6907

7

6907fa

THEORY OF OPERATIO

The LTC6907 is a precision, resistor programmable oscil-

lator (see Block Diagram). It generates a square wave at

the OUT pin with a period directly proportional to the value

and controls the oscillator frequency to achieve the high-

est possible accuracy. In equilibrum, this circuit ensures

is proportional to the master oscillator frequency, so we

have the relationship:

(1)

Solving for the oscillator period:

t

V

I

C

OSC

OSC

SET

SET

OSC

==

1

ƒ

•

(2)

This is the fundamental equation for the LTC6907. It holds

regardless of how the SET pin is driven. When a resistor,

the relationship:

V

I

R

SET

SET

SET

=

(3)

so

tR

C

OSC

OSC

SET

OSC

==

1

ƒ

•

(4)

The period and frequency are determined exclusively by

voltage.

The digital dividers shown in the Block Diagram further

divide the master oscillator frequency by 1, 3 or 10

producing:

ƒ

ƒ

OUT

OSC

N

=

(5)

and

(6)

Table 1 gives specific frequency and period equations for

the LTC6907. The Applications Information section gives

further detail and discusses alternative ways of setting the

LTC6907 output frequency.

Table 1. Output Frequency Equations

PART NUMBER

FREQUENCY

PERIOD

DIVIDER RATIOS

LTC6907

ƒ

OUT

SET

MHz

N

k

R

=

⎛

⎝⎜

⎞

⎠⎟

4

50

•

tN

ns

R

k

OUT

SET

=

⎛

⎝⎜

⎞

⎠⎟

••

250

50

N

Open

=

⎧

⎨

⎪

⎩

⎪

=

=

=

+

10

3

1

,

,

,

DIV Pin

V

DIV Pin

DIV Pin

GND