LTC6990_15 Datasheet, PDF (12/30 Pages) Linear Technology – Voltage Controlled Silicon Oscillator

LTC6990
OPERATION
is not stable, it will increase the start-up time as the con-
verter waits for a stable result. Therefore, capacitance on
the DIV pin should be minimized so it will settle quickly.
Less than 100pF will not affect performance.
Output Enable
The OE pin controls the state of the LTC6990’s output as
seen on the OUT pin. Pulling the OE pin high enables the
oscillator output. Pulling it low disables the output. When
the output is disabled, it is either held low or placed in
a high impedance state as dictated by the Hi-Z bit value
(determined by the DIVCODE as described earlier). Table 2
summarizes the output control states.
Table 2. Output States
OE Pin
Hi-Z
1
X
0
1
0
0
OUT
Enabled, Output is Active
Disabled, Output is Hi-Z
Disabled, Output is Held Low
Figure 3 illustrates the timing for the OE function when
Hi-Z = 0. When OE is low, the output is disabled and OUT
is held low. Bringing OE high enables the output after a
delay, t_ENABLE, which synchronizes the enable to eliminate
sliver pulses and guarantee the correct width for the first
pulse. If N_DIV = 1 or 2 this delay will be no longer than
the output period, t_OUT. If N_DIV > 2 the delay is limited to
twice the internal master oscillator period (or 2 • t_MASTER).
Forcing OE low will bring OUT low after a propagation
delay, t_PD. If the output is high when OE falls, the output
pulse will be truncated.
As shown in Figure 4, setting Hi-Z = 1 places the output in
a high-impedance state when OE = 0. This feature allows
for “wired-OR” connections of multiple devices. Driving OE
high enables the output. The output will usually be forced
low during this time, although it is possible for OUT to
transition directly from high-impedance to a high output,
depending on the timing of the OE transition relative to
the internal oscillator. Once high, the first output pulse
will have the correct width (unless truncated by bringing
OE low again).
OE
t_PD
OUT
t_ENABLE
t_OUT
t_ENABLE
Figure 3. OE Timing Diagram (Hi-Z = 0)
12
OE
t_PD
OUT
Hi-Z
t_ENABLE
t_OUT
t_PD
t_PD
t_ENABLE
Figure 4. OE Timing Diagram (Hi-Z = 1)
For more information www.linear.com/LTC6990
t_PD
6990 F03
t_PD
6990 F04
6990fc