Datasheet

LTC6902
7
6902f
or it should be surrounded by a ground shield to prevent
excessive coupling from other PCB traces.
OUT1, OUT2, OUT3, OUT4 (Pins 4, 5, 6 and 7): Oscillator
Outputs. These pins can drive 5kΩ and/or 10pF loads.
Larger loads may cause inaccuracies due to supply bounce
at high frequencies.
GND (Pin 8): Ground. Should be tied to a ground plane for
best performance.
SET (Pin 10): Frequency Setting Resistor Input. The value
of the resistor (R
SET
) connected between Pin 10 and V
+
determines the oscillator frequency. The voltage on this
pin is held at approximately 1.13V below the V
+
voltage.
For best performance, use a precision metal film resistor
with a value between 10k and 2M and limit the capacitance
on this pin to less than 10pF.
UU
U
PI FU CTIO S
MOD (Pin 9): Spread Spectrum Frequency Modulation
Setting Resistor Input. The value of the resistor (R
MOD
)
connected between Pin 9 and V
+
determines the amount
of frequency modulation. The output frequency is always
modulated down from the frequency set by the R
SET
resis-
tor. For best performance, use a precision metal film resis-
tor with a value between 10k and 2M and limit the
capacitance on this pin to less than 10pF. The voltage on
this pin is not static. Limiting the capacitance on this pin
is important for the part to perform properly. To disable
the modulation, connect this pin to GND. Grounding the
MOD pin disables the modulation and shuts down the
modulation circuitry to save power. Leaving the pin open
to disable the modulation is not recommended. While
leaving the pin open, R
MOD
≈ ∞, gives the mathematical
result of 0% modulation, the open pin is susceptible to
external noise coupling that can effect frequency accuracy.
BLOCK DIAGRA
W
–
+
–
+
1
10
9
I
SET
I
SET
MOD
8
GND
CURRENT MIRROR
I
MASTER
I
SET
G = 1
I
MOD
I
MOD
V
BIAS
V
+
– V
SET
= 1.1V ±25%
MASTER OSCILLATOR
PROGRAMMABLE
DIVIDER ÷N
(N = 1, 10 OR 100)
÷3200
6902 BD
9-BIT PRBS
GENERATOR
V
SET
REF
MULTIPLYING
DAC
7
MULTIPHASE
CIRCUIT
SELECTS BETWEEN
MULTIPHASE
OPTIONS AND ÷M
2-PHASE (M = 1)
3-PHASE (M = 3)
4-PHASE (M = 4)
I
MOD
SET
V
+
R
SET
R
MOD
+
–
I
MASTER
(V
+
– V
SET
)
f
MASTER
= 10MHz • 20kΩ •
2 3
4
DIV PH
OUT1
5
OUT2
6
OUT3
7
OUT4