Datasheet

LTC6906
7
6906fc
The LTC6906 is a precision, resistor programmable oscilla-
tor (see the Block Diagram). It generates a square wave at
the OUT pin with a period directly proportional to the value
of an external resistor, R
SET
. A feedback circuit measures
and controls the oscillator frequency to achieve the high-
est possible accuracy. In equilibrium, this circuit ensures
that the current in the SET pin, I
SET
, is balanced by I
FB
.
I
FB
is proportional to the master oscillator frequency, so
we have the relationship:
I
SET
= I
FB
= V
SET
• ƒ
OSC
• C
OSC
(1)
where C
OSC
is a precision internal capacitor:
C
OSC
= 10pF for the LTC6906
Solving for the oscillator period:
t
OSC
=
1
ƒ
OSC
=
V
SET
I
SET
•C
OSC
(2)
This is the fundamental equation for the LTC6906. It holds
regardless of how the SET pin is driven. When a resistor,
R
SET
, is connected from the SET pin to ground, we have
the relationship:
V
SET
I
SET
= R
SET
(3)
so
t
OSC
=
1
ƒ
OSC
= R
SET
•C
OSC
(4)
THEORY OF OPERATION
The period and frequency are determined exclusively by
R
SET
and the precision internal capacitor. Importantly, the
value of V
SET
is immaterial, and the LTC6906 maintains
its accuracy even though V
SET
is not a precision reference
voltage.
The digital dividers shown in the Block Diagram further
divide the master oscillator frequency by 1, 3 or 10 pro-
ducing:
ƒ
OUT
=
ƒ
OSC
N
(5)
and
t
OUT
= N • t
OSC
(6)
Table 1 gives specific frequency and period equations for
the LTC6906. The Applications Information section gives
further detail and discusses alternative ways of setting the
LTC6906 output frequency.
Table 1. Output Frequency Equations
PART NUMBER FREQUENCY PERIOD DIVIDER RATIOS
LTC6906
ƒ
OUT
=
1MHz
N
•
100k
R
SET
⎛
⎝
⎜
⎜
⎞
⎠
⎟
⎟
t
OUT
=N•1μs•
R
SET
100k
⎛
⎝
⎜
⎞
⎠
⎟
N =
10,
3,
1,
⎧
⎨
⎪
⎩
⎪
DIV Pin = V
+
DIV Pin = Open
DIV Pin = GND