
LTC6902
6
6902f
QUICK DESIG GUIDE
U
Step 3. Calculating the RSET Resistor Value
The RSET resistor, the multiphase mode and the divider
setting set the output frequency (fOUT) for constant fre-
quency applications. For SSFM applications, the maxi-
mum frequency excursion (fMAX) is equal to fOUT.
Rk
MHz
NM f
N
Open
V
M
H
Open
HV
SET
OUT
=
=
=
=
+
20
10
100
10
1
4
3
1
DIV Pin
V
DIV Pin
0
(4 - Phase Output) PH Pin
V
(3 - Phase Output) P Pin
(2 - Phase Output) P Pin
0
Step 4. Calculating the RMOD Resistor Value
(Note: For constant frequency applications RMOD is not
required. Disable SSFM by connecting the MOD pin to
GND)
R
Spreading Percentage
MOD
SET
= 20
where the Spreading Percentage is defined by the
following:
Spreading Percentage
ff
f
MAX
MIN
MAX
= 100
–
where fMAX is the highest frequency excursion (set by the
RSET value calculated in Step 3) and fMIN is the lowest
frequency excursion.
Example
For a 4-phase, 250kHz clock with 40% spreading:
Connect PH Pin to V+
→ Selects 4-Phase Mode, M = 4
Leave DIV Pin Open
→ N = 10
RSET = 20k → Sets fOUT = fMAX = 250kHz
RMOD = 10k → Sets Spreading to 40%
UU
U
PI FU CTIO S
V+ (Pin 1): Supply Voltage ( 2.7V
≤ V+ ≤ 5.5V). The supply
should be kept free from noise and ripple. It should be
bypassed directly to a ground plane with a 0.1
F capacitor
placed as close to the pin as possible.
DIV (Pin 2): Divider Setting Input. This three-state input
selects among three divider settings determining the value
of N in the frequency equation. Pin 2 should be tied to GND
for the
÷1 setting, the highest frequency range. Floating
Pin 2, leaving it open, divides the master oscillator by 10.
Tie Pin 2 to V+ for the
÷100 setting, the lowest frequency
range. To detect a floating DIV pin, the LTC6902 places the
pin at the midsupply point with active circuitry. Therefore,
driving the DIV pin high requires sourcing approximately
2
A. Similarly, driving the DIV pin low requires sinking
2
A. When the DIV pin is floated, it should be bypassed by
a 1nF capacitor to GND or it should be surrounded by a
ground shield to prevent excessive coupling from other
PCB traces.
PH (Pin 3): Phase Setting Input. This three-state input
selects among three multiphase options. This sets the
outputs to produce 2-phase, 3-phase or 4-phase signals.
It also sets the value of M in the frequency equation. Pin 3
should be tied to GND for the 2-phase setting. This is the
highest frequency range with M set to 1. Floating Pin 3,
leaving it open, selects the 3-phase setting. This also sets
M to 3. Tie Pin 3 to V+ for the 4-phase setting. This is the
lowest frequency range as M is set to 4. To detect a floating
PH pin, the LTC6902 places the pin at the midsupply point
with active circuitry. Therefore, driving the PH pin high
requires sourcing approximately 2
A. Similarly, driving
the PH pin low requires sinking 2
A. When the PH pin is
floated, it should be bypassed by a 1nF capacitor to GND