LTC6902IMS#TR Linear Technology, LTC6902IMS#TR Datasheet - Page 8

• Manufacturer Part Number: LTC6902IMS#TR
• Description: IC OSC MULTIPHASE PREC LP 10MSOP
• Manufacturer: Linear Technology
• Type: Oscillator, Siliconr

Specifications of LTC6902IMS#TR

• Frequency: 20MHz
• Voltage - Supply: 2.7 V ~ 5.5 V
• Current - Supply: 2.5mA
• Operating Temperature: -40°C ~ 85°C
• Package / Case: 10-MSOP, Micro10™, 10-uMAX, 10-uSOP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Count: -

THEORY OF OPERATIO
LTC6902
As shown in the Block Diagram, the LTC6902’s master
oscillator is controlled by the ratio of the voltage between
the V
the master oscillator, I
frequency modulation (SSFM) is disabled, I
strictly determined by the V
current. When SSFM is enabled, the current I
lation current) is subtracted from the I
determine the I
current is maximally at I
less than I
this way the frequency of the master oscillator is modu-
lated to produce a frequency that is always less than or
equal to the frequency set by the I
The voltage on the SET pin is forced to approximately 1.1V
below V
This voltage is accurate to 8% at a particular input
current and supply voltage (see Figure 2). The R
resistor, connected between the V
together the (V
allows the parts to attain excellent frequency accuracy
regardless of the precision of the SET pin voltage. The
LTC6902 is optimized for use with R
10k and 2M. This corresponds to master oscillator fre-
quencies between 100kHz and 20MHz. Additionally, the
MOD pin’s voltage tracks the SET pin’s voltage. The R
resistor connected between the V
locks together the MOD pin voltage variation and the I
current to once more yield excellent accuracy.
8
+
and SET pins (V
+
by the PMOS transistor and its gate bias voltage.
SET
1.4
1.3
1.2
1.1
1.0
0.9
0.8
Figure 2. V
0.1
by a value determined by the I
+
MASTER
– V
SET
V
1
+
+
= 3V
MASTER
) voltage and the current I
+
– V
current value. Here the I
SET
– V
I
RES
SET
V
+
10
SET
+
= 5V
( A)
but more often than not it is
Variation with I
. When the spread spectrum
– V
) and the current entering
+
SET
and MOD pins similarly
SET
100
+
SET
and SET pins, locks
voltage and the I
current.
U
69012 F02
resistors between
1000
SET
RES
MOD
MOD
current to
MASTER
SET
value. In
(modu-
MASTER
. This
MOD
MOD
SET
SET
is
The master oscillator’s output is connected to the pro-
grammable divider. The output of the programmable
divider is then connected to the multiphase circuit with its
four outputs directly connected to output drivers. The final
output frequency is determined by the R
the programmable divider setting and the multiphase
mode selected. The formula for setting the output fre-
quency, f
where:
When the spread spectrum frequency modulation (SSFM)
is disabled, the frequency f
quency. When SSFM is enabled, f
output frequency with the R
ing the minimum output frequency.
The programmable divider divides the master oscillator
signal by 1, 10 or 100. The divide-by value is determined
by the state of the DIV input (Pin 2). Tie DIV to GND or drive
it below 0.5V to select 1. This is the highest frequency
range, with the master output frequency passed directly to
the multiphase circuit. The DIV pin may be floated or
driven to midsupply to select 10, the intermediate fre-
quency range. The lowest frequency range, 100, is se-
lected by tying DIV to V
Figure 3 shows the relationship between R
setting and output frequency, including the overlapping
frequency ranges near 100kHz and 1MHz.
The multiphase circuit generates outputs that are either
2-, 3- or 4-phase waveforms. To generate the 3- and
4-phase output signals, the output from the programmable
N
M
f
OUT
100
10
1
3
1
4
OUT
10
N M
, is below:
MHz
•
DIV Pin V
DIV Pin
DIV Pin 0
(4 - Phase Output) PH Pin V
(3 - Phase Output) P Pin
(2 - Phase Output) P Pin 0
20
R
+
SET
Open
k
or driving it to within 0.4V of V
V
MOD
OUT
resistor value determin-
is the final output fre-
H
H
OUT
SET
is the maximum
resistor value,
Open
V
SET
, divider
6902f
+
.