LTC6906 Linear Technology, LTC6906 Datasheet - Page 7

• Manufacturer Part Number: LTC6906
• Description: Micropower
• Manufacturer: Linear Technology

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THEORY OF OPERATIO
The LTC6906 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
of an external resistor, R
and controls the oscillator frequency to achieve the high-
est possible accuracy. In equilibrum, this circuit ensures
that the current in the SET pin, I
is proportional to the master oscillator frequency, so we
have the relationship:
Where C
Solving for the oscillator period:
This is the fundamental equation for the LTC6906. It holds
regardless of how the SET pin is driven. When a resistor,
R
the relationship:
Table 1. Output Frequency Equations
SET
I
C
t
SET
V
I
OSC
OSC
SET
SET
, is connected from the SET pin to ground, we have
PART NUMBER
= I
LTC6906
= 10pF for the LTC6906
=
OSC
=
FB
ƒ
R
OSC
= V
SET
is a precision internal capacitor:
1
SET
=
V
I
• ƒ
SET
SET
OSC
SET
•
C
• C
. A feedback circuit measures
OSC
ƒ
OSC
OUT
SET
FREQUENCY
=
, is balanced by I
1
MHz
N
U
•
⎛
⎜
⎝
100
R
SET
k
⎞
⎟
⎠
FB
. I
(1)
(2)
(3)
FB
t
so
The period and frequency are determined exclusively by
R
value of V
accuracy even though V
voltage.
The digital dividers shown in the Block Diagram further
divide the master oscillator frequency by 1, 3 or 10
producing:
and
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.
OUT
SET
t
t
ƒ
OUT
=
OSC
OUT
N
and the precision internal capacitor. Importantly, the
PERIOD
•
1
= N • t
µ
=
=
s
SET
•
ƒ
ƒ
⎛
⎜
⎝
OSC
R
100
1
OSC
is immaterial, and the LTC6906 maintains its
N
OSC
SET
k
⎞
⎟
⎠
=
R
SET
•
SET
C
OSC
is not a precision reference
N
=
DIVIDER RATIOS
⎧
⎪
⎨
⎪
⎩
10
1
3
,
,
,
DIV Pin V
DIV Pin
DIV Pin GND
LTC6906
=
=
=
Open
+
6906fa
7
(4)
(5)
(6)