Datasheet

LTC6602
17
6602fc
APPLICATIONS INFORMATION
Figure 6. Current Controlled Clock Frequency
Figure 7. Voltage Controlled Clock Frequency
R
BIAS
6602 F06
I
CONTROL
f
CLK
= 10k • (494.1MHz/1.17V) • I
CONTROL
(A)
R
BIAS
6602 F07
V
CONTROL
f
CLK
= 494.1MHz • (10k/R
BIAS
) • (1 – V
CONTROL
/1.17V)
R
BIAS
+
–
Preserving Oscillator Accuracy
The oscillator is sensitive to transients on the positive
supply. The IC should be soldered to the PC board and
the PCB layout should include a 0.1µF ceramic capacitor
between V+
A
(Pin 2) and ground, as close as possible to
the IC to minimize inductance. The PCB layout should also
include an additional 0.1µF ceramic capacitor between
V+
D
(Pin 16) and ground. Avoid parasitic capacitance on
R
BIAS
(Pin 4) and avoid routing noisy signals near R
BIAS
.
Use a ground plane connected to Pin 14 and the Exposed
Pad (Pin 25).
Alternative Methods of Setting the Clock Frequency of
the LTC6602
The oscillator may be programmed by any method that
sinks a current out of the R
BIAS
pin. The circuit in Figure 6
sets the clock frequency by using a programmable current
source and in the expression for f
CLK
, the resistor R
BIAS
is replaced by the ratio of 1.17V/I
CONTROL
. Because the
voltage of the R
BIAS
pin is approximately 1.17V ±5%, the
Figure 6 circuit is less accurate than if a resistor controls
the clock frequency.
Figure 7 shows the LTC6602’s oscillator configured as
a VCO. A voltage source is connected in series with the
R
BIAS
resistor. The clock frequency, f
CLK
, will vary with
V
CONTROL
. Again, this circuit decouples the relationship
between the current out of the R
BIAS
pin and the voltage
of the R
BIAS
pin; the frequency accuracy will be degraded.
The clock frequency, however, will increase monotonically
with decreasing V
CONTROL
.
Operation Using an External Clock
The LTC6602 may be clocked by an external oscillator
for tighter bandwidth control by pulling CLKCNTL (Pin 5)
to ground and driving a clock into CLKIO (Pin 15). If an
external clock is used, the R
BIAS
resistor is still necessary.
The value of R
BIAS
must be no larger than the value that
would be required for using the internal oscillator. For
example, a 100k resistor would program the internal oscil-
lator for 49.41MHz, so an external oscillator frequency of
49.41MHz would require an R
BIAS
resistance of no more
than 100k. If the value of R
BIAS
is too large, the filters will
not receive a large enough bias current, possibly causing
errors due to insufficient settling.