LTC6907HS6 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC6907HS6 Datasheet PDF : 12 Pages
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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
of an external resistor, RSET. A feedback circuit measures
and controls the oscillator frequency to achieve the high-
est possible accuracy. In equilibrum, this circuit ensures
that the current in the SET pin, ISET, is balanced by IFB. IFB
is proportional to the master oscillator frequency, so we
have the relationship:
ISET = IFB = VSET • ƒOSC • COSC
(1)
Where COSC is a precision internal capacitor:
COSC = 5pF for the LTC6907
Solving for the oscillator period:
tOSC =
1
ƒOSC
= VSET
ISET
• COSC
(2)
This is the fundamental equation for the LTC6907. It holds
regardless of how the SET pin is driven. When a resistor,
RSET, is connected from the SET pin to ground, we have
the relationship:
VSET
ISET
=
RSET
(3)
LTC6907
so
tOSC =
1
ƒOSC
= RSET • COSC
(4)
The period and frequency are determined exclusively by
RSET and the precision internal capacitor. Importantly, the
value of VSET is immaterial, and the LTC6907 maintains its
accuracy even though VSET 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
producing:
ƒOUT
=
ƒOSC
N
(5)
and
tOUT = N • tOSC
(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
LTC6907
ƒOUT
=
4MHz
N
•
⎛
⎝⎜
50k
RSET
⎞
⎠⎟
PERIOD
tOUT
=
N
•
250ns
•
⎛
⎝⎜
RSET
50k
⎞
⎠⎟
DIVIDER RATIOS
⎧10, DIV Pin = V +
N = ⎨⎪3, DIV Pin = Open
⎩⎪1, DIV Pin = GND
6907fa
7
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