LTC6991 データシートの表示（PDF） - Analog Devices
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LTC6991 Datasheet PDF : 24 Pages
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LTC6991
OPERATION
The LTC6991 is built around a master oscillator with a
1MHz maximum frequency. The oscillator is controlled
by the SET pin current (ISET) and voltage (VSET), with a
1MHz • 50k conversion factor that is accurate to ±0.8%
under typical conditions.
fMASTER
=
1
tMASTER
= 1MHz
• 50kΩ
•
ISET
VSET
A feedback loop maintains VSET at 1V ±30mV, leaving ISET
as the primary means of controlling the output frequency.
The simplest way to generate ISET is to connect a resistor
(RSET) between SET and GND, such that ISET = VSET/RSET .
The master oscillator equation reduces to:
fMASTER
=
1
tMASTER
=
1MHz • 50kΩ
RSET
From this equation, it is clear that VSET drift will not affect
the output frequency when using a single program resis-
tor (RSET). Error sources are limited to RSET tolerance and
the inherent frequency accuracy ∆fOUT of the LTC6991.
RSET may range from 50k to 800k (equivalent to ISET
between 1.25µA and 20µA).
Before reaching the OUT pin, the oscillator frequency
passes through a fixed ÷1024 divider. The LTC6991
also includes a programmable frequency divider which
can further divide the frequency by 1, 8, 64, 512, 4096,
215, 218 or 221. The divider ratio NDIV is set by a resistor
divider attached to the DIV pin.
fOUT
=
1MHz • 50kΩ
1024 • NDIV
•
ISET
VSET
,
or
tOUT
=
1
fOUT
=
NDIV
50kΩ
•
VSET
ISET
• 1.024ms
with RSET in place of VSET/ISET the equation reduces to:
tOUT
=
NDIV • RSET
50kΩ
• 1.024ms
DIVCODE
The DIV pin connects to an internal, V+ referenced 4-bit
A/D converter that determines the DIVCODE value.
DIVCODE programs two settings on the LTC6991:
1. DIVCODE determines the output frequency divider set-
ting, NDIV .
2. DIVCODE determines the polarity of the RST and OUT
pins, via the POL bit.
VDIV may be generated by a resistor divider between V+
and GND as shown in Figure 1.
2.25V TO 5.5V
V+
LTC6991
R1
DIV
R2
GND
6991 F01
Figure 1. Simple Technique for Setting DIVCODE
Table 1 offers recommended 1% resistor values that accu-
rately produce the correct voltage division as well as the
corresponding NDIV and POL values for the recommended
resistor pairs. Other values may be used as long as:
1. The VDIV/V+ ratio is accurate to ±1.5% (including resis-
tor tolerances and temperature effects)
2. The driving impedance (R1||R2) does not exceed
500kΩ.
If the voltage is generated by other means (i.e., the output
of a DAC) it must track the V+ supply voltage. The last
column in Table 1 shows the ideal ratio of VDIV to the
supply voltage, which can also be calculated as:
VDIV
V+
=
DIVCODE + 0.5
16
± 1.5%
For example, if the supply is 3.3V and the desired DIVCODE
is 4, VDIV = 0.281 • 3.3V = 928mV ± 50mV.
Figure 2 illustrates the information in Table 1, showing
that NDIV is symmetric around the DIVCODE midpoint.
For more information www.linear.com/LTC6991
6991fc
9
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