Datasheet

LTC6990
13
6990fc
For more information www.linear.com/LTC6990
OPERATION
Changing DIVCODE After Start-Up
Following start-up, the A/D converter will continue
monitoring V
DIV
for changes. Changes to DIVCODE will
be recognized slowly, as the LTC6990 places a priority on
eliminating any “wandering” in the DIVCODE. The typical
delay depends on the difference between the old and
new DIVCODE settings and is proportional to the master
oscillator period.
t
DIVCODE
= 16 •(∆DIVCODE + 6)• t
MASTER
A change in DIVCODE will not be recognized until it is
stable, and will not pass through intermediate codes. A
digital filter is used to guarantee the DIVCODE has settled
to a new value before making changes to the output. Then
the output will make a clean (glitchless) transition to the
new divider setting.
Start-Up Time
When power is first applied to the LTC6990 the power-on
reset (POR) circuit will initiate the start-up time, t
START
.
The OUT pin is floated (high-impedance) during this time.
The typical value for t
START
ranges from 0.5ms to 8ms
depending on the master oscillator frequency (indepen-
dent of N
DIV
):
t
START(TYP)
= 500 • t
MASTER
The start-up time may be longer if the supply or DIV
pin voltages are not stable. For this reason, it is recom-
mended to
minimize the capacitance on the DIV pin so it
will properly track V
+
.
Figure 5. DIVCODE Change from 5 to 2 Figure 6. Typical Start-Up
100µs/DIV
6990 F05
DIV
1V/DIV
OUT
1V/DIV
V
+
= 3.3V
R
SET
= 200k
576µs
100µs/DIV
6990 F06
V
+
1V/DIV
OUT
1V/DIV
V
+
= 2.5V
DIVCODE = 4
R
SET
= 50k
OUTPUT CONNECTED
TO 1.25V THROUGH 25k
TO SHOW Hi-Z
470µs