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LTC6990
15
6990fc
For more information www.linear.com/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
procedureisasimpletwostepprocess.FirstselecttheNDIV
value and then calculate the value for the RSET resistor.
Alternatively, Linear Technology offers the easy to use
TimerBlox Designer tool to quickly design any LTC6990
based circuit. Download the free TimerBlox Designer
software at www.linear.com/timerblox.
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
(generallyrecommended).Forfasterstart-upordecreased
jitter,chooseahigherNDIVsetting.Alternatively,useTable1
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 NDIVinclude4,8,16,and32.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.
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