LTC6990CS6-PBF(V2) View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
LTC6990CS6-PBF Datasheet PDF : 28 Pages
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LTC6990
APPLICATIONS INFORMATION
Start-Up Behavior
When first powered up, the output is high impedance. If
the output is enabled (OE = 1) at the end of the start-up
time, the output will go low for one tMASTER cycle (or half
a tOUT cycle if NDIV < 4) before the first rising edge. If the
output is disabled (OE = 0) at the end of the start-up time,
the output will drop to a low output if the Hi-Z bit = 0, or
simply remain floating if Hi-Z = 1.
Basic Fixed Frequency Operation
The simplest and most accurate method to program the
LTC6990 for fixed frequency operation is to use a single
resistor, RSET, between the SET and GND pins. The design
procedure is a simple two step process. First select the NDIV
value and then calculate the value for the RSET resistor.
Step 1: Selecting the NDIV Frequency Divider Value
As explained earlier, the voltage on the DIV pin sets the
DIVCODE which determines both the Hi-Z bit and the
NDIV value. For a given output frequency, NDIV should be
selected to be within the following range.
62.5kHz
fOUT
≤
NDIV
≤
1MHz
fOUT
(1a)
To minimize supply current, choose the lowest NDIV value
(generally recommended). For faster start-up or decreased
jitter, choose a higher NDIV setting. Alternatively, use Table 1
as a guide to select the best NDIV value for the given ap-
plication. After choosing the value for NDIV, use Table 1 to
select the proper resistor divider or VDIV/V+ ratio to apply
to the DIV pin.
Step 2: Calculate and Select RSET
The final step is to calculate the correct value for RSET
using the following equation.
RSET
=
1MHz • 50k
NDIV • fOUT
(1b)
Select the standard resistor value closest to the calculated
value.
Example: Design a 20kHz Oscillator with Minimum
Power Consumption
Step 1: Selecting the NDIV Frequency Divider Value
First, choose an NDIV value that meets the requirements
of Equation (1a).
3.125 ≤ NDIV ≤ 50
Potential settings for NDIV include 4, 8, 16, and 32. NDIV = 4
is the best choice, as it minimizes supply current by using
a large RSET resistor. Using Table 1, choose the R1 and R2
values to program DIVCODE to either 2 or 13, depending
on the desired behavior when the output is disabled.
Step 2: Select RSET
Calculate the correct value for RSET using Equation (1b).
RSET
=
1MHz • 50k
4 • 20kHz
=
625k
Since 625k is not available as a standard 1% resistor,
substitute 619k if a 0.97% frequency shift is acceptable.
Otherwise, select a parallel or series pair of resistors such
as 309k and 316k to attain a more precise resistance.
Frequency Modulated Operation (Voltage-Controlled
Oscillator)
Operating the LTC6990 as a voltage-controlled oscillator in
its simplest form is achieved with one additional resistor. As
shown in Figure 11, voltage VCTRL sources/sinks a current
through RVCO to vary the ISET current, which in turn modu-
lates the output frequency as described in Equation (2).
fOUT
=
1MHz • 50k
NDIV • RVCO
•
⎛
⎝⎜
1+
RVCO
RSET
−
VCTRL
VSET
⎞
⎠⎟
(2)
V+
VCTRL
OE
OUT
LTC6990
GND
V+
RVCO
SET
RSET
DIV
6990 F08
V+
C1
0.1μF R1
R2
Figure 11. Voltage Controlled Oscillator
6990f
15
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