Datasheet

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 t
MASTER
cycle (or half
a t
OUT
cycle if N
DIV
< 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, R
SET
, between the SET and GND pins. The design
procedure is a simple two step process. First select the N
DIV
value and then calculate the value for the R
SET
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 N
DIV
Frequency Divider Value
As explained earlier, the voltage on the DIV pin sets the
DIVCODE which determines both the Hi-Z bit and the
N
DIV
value. For a given output frequency, N
DIV
should be
selected to be within the following range.
62.5kHz
f
OUT
≤ N
DIV
≤
1MHz
f
OUT
(1a)
To minimize supply current, choose the lowest N
DIV
value
(generally recommended). For faster start-up or decreased
jitter, choose a higher N
DIV
setting. Alternatively, use Table 1
as a guide to select the best N
DIV
value for the given ap-
plication. After
choosing the value for N
DIV
, use Table 1 to
select the proper resistor divider or V
DIV
/V
+
ratio to apply
to the DIV pin.
Step 2: Calculate and Select R
SET
The final step is to calculate the correct value for R
SET
using the following equation.
R
SET
=
1MHz • 50k
N
DIV
• f
OUT
(1b)
Select the standard resistor value closest to the calculated
value.
Example: Design a 20kHz Oscillator with Minimum
Power Consumption
Step 1: Selecting the N
DIV
Frequency Divider Value
First, choose an N
DIV
value that meets the requirements
of Equation (1a).
3.125 ≤ N
DIV
≤ 50
Potential settings for N
DIV
include 4, 8, 16, and 32. N
DIV
= 4
is the best choice, as it minimizes supply current by using
a large R
SET
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 R
SET
Calculate the correct value for R
SET
using Equation (1b).
R
SET
=
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.