Datasheet

LTC1799
5
1799fc
pin FuncTions
V
+
(Pin 1): Voltage Supply (2.7V ≤ V
+
≤ 5.5V). This sup-
ply must be kept free from noise and ripple. It should be
bypassed directly to a ground plane with a 0.1µF capacitor.
GND (Pin 2): Ground. Should be tied to a ground plane
for best performance.
SET (Pin 3): Frequency-Setting Resistor Input. The value
of the resistor connected between this pin and V
+
deter-
mines the oscillator frequency. The voltage on this pin is
held by the LTC1799 to approximately 1.13V below the
V
+
voltage. For best performance, use a precision metal
film resistor with a value between 10k and 200k and limit
the capacitance on this pin to less than 10pF.
DIV (Pin 4): Divider-Setting Input. This three-state input
selects among three divider settings, determining the
value of N in the frequency equation. Pin 4 should be tied
to GND for the ÷1 setting, the highest frequency range.
Floating Pin 4 divides the master oscillator by 10. Pin 4
should be tied to V
+
for the ÷100 setting, the lowest fre-
quency range. To detect a floating DIV pin, the LTC1799
attempts to pull the pin toward midsupply. This is realized
with two internal current sources, one tied to V
+
and Pin
4 and the other one tied to ground and Pin 4. Therefore,
driving the DIV pin high requires sourcing approximately
5µA. Likewise, driving DIV low requires sinking 5µA.
When Pin 4 is floated, preferably it should be bypassed
by a 1nF capacitor to ground or it should be surrounded
by a ground shield to prevent excessive coupling from
other PCB traces.
OUT (Pin 5): Oscillator Output. This pin can drive 5kΩ
and/or 10pF loads. Larger loads may cause inaccuracies
due to supply bounce at high frequencies. Transients will
not cause latchup if the current into/out of the OUT pin is
limited to 50mA.
block DiagraM
–
+
+
–
1
3
GAIN = 1
V
+
V
BIAS
I
RES
I
RES
R
SET
SET
GND
MASTER OSCILLATOR
PROGRAMMABLE
DIVIDER
(÷1, 10 OR 100)
V
RES
= 1.13V ±25%
(V
+
– V
SET
)
I
RES
(V
+
– V
SET
)
ƒ
MO
= 100MHz • kΩ •
THREE-STATE
INPUT DETECT
GND
V
+
5µA
1799 BD
5µA
OUT
DIVIDER
SELECT
5
DIV
42