DS1090U-2 MAXIM [Maxim Integrated Products], DS1090U-2 Datasheet - Page 7

• Manufacturer Part Number: DS1090U-2
• Description: Low-Frequency, Spread-Spectrum EconOscillator
• Manufacturer: MAXIM [Maxim Integrated Products]

The DS1090 is a center-dithered, spread-spectrum sili-
con oscillator for use as an external clock in reduced-
EMI applications. With a combination of factory-
programmed prescalers and a user-selected external
resistor, output frequencies from 125kHz to 8MHz can
be achieved. The output center frequency can be
dithered by selecting the desired dither rate and ampli-
tude with discrete inputs J0, J1, JC0, and JC1.
The DS1090 contains four basic circuit blocks: master
oscillator, factory-programmed prescaler, dither gener-
ator, and the voltage-bias circuit that provides the feed-
back path to the master oscillator for frequency control
and dithering functions.
The master oscillator is programmable in the applica-
tion by the use of an external resistor (R
ground (GND). Resistor values of 45kΩ to 91kΩ vary
the square-wave output frequency of the voltage-con-
trolled master oscillator (f
4MHz (see Figure 1).
The master oscillator (Hz) frequency can be stated as
Figure 1. Master Oscillator Frequency
9
8
7
6
5
4
3
40
MASTER OSCILLATOR FREQUENCY vs.
EXTERNAL RESISTOR SELECTION
f
MOSC
50
R
SET
Detailed Description
60
≅
RESISTANCE (kΩ)
3 6461
.
Re
MOSC
70
sistor
E
+
) from 8MHz down to
Master Oscillator
80
11
Low-Frequency, Spread-Spectrum
_____________________________________________________________________
90
SET
) tied to
The prescaler divides the frequency of the master oscil-
lator by 1, 2, 4, 8, 16, or 32 to generate the square-
wave output clock (f
is an ordering option.
Spread-spectrum functionality is achieved by a user-
configurable divider (determines dither rate), a triangle
generator, and a user-configurable dither amplitude cir-
cuit (see Block Diagram).
The input to the triangle-wave generator is derived from
the internal master oscillator and is fed through a user-
configurable divider. The settings of control pins JC0
and JC1 determine this dither rate divisor setting (see
Table 1), dividing the master clock by 4, 8, 16, or 32.
The clock signal is further divided by 128 in the
triangle-wave generator, which results in a triangle-
wave signal of either 1/512th, 1/1024th, 1/2048th, or
1/4096th of the master oscillator (f
upon the user’s divisor setting.
The dithering frequency can be also expressed as the
result of
where Divisor is 4, 8, 16, or 32.
Figure 2. Center Frequency Dither Diagram
Table 1. Dither Rate Divisor Settings
JC1
(+ 1, 2, or 4% of f
(- 1, 2, or 4% of f
0
0
1
1
Programmed f
JC0
0
1
0
1
MOSC
MOSC
MOSC
Factory-Programmed Prescaler
DITHERING PERCENTAGE
)
)
f
MOD
EconOscillator
OSC
=
F
f
f
F
MOSC
MOSC
MOSC
Divisor
MOSC
(f
). This divisor is factory-set and
MOSC
f
MOD
1
f
MOSC
/ 2048
/ 4096
TIME
/ 1024
/ 512
/n)
×128
Dither Generator
IF DITHER AMOUNT = 0%
MOD
), depending
SETTING
DIVISOR
DITHER
AMOUNT
(2, 4, OR 8%)
16
32
4
8
7