LTC6902 datasheet(6/16 Pages) AD

6 / 16 page

LTC6908-1/LTC6908-2

Rev C

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BLOCK DIAGRAM

OUT1

690812 BD

OUT2

V+

GND

SET

MOD 4

GND

COMPLEMENTARY

QUADRATURE

OUTPUTS

MUTE OUTPUT

UNTIL STABLE

POR

1-POLE

LPF

90/180

DIVIDE BY

16/32/64

3-STATE

INPUT DECODER

DETECT

CLOCK INPUT

WHEN A CLOCK SIGNAL IS PRESENT AT THE

MOD INPUT, DISABLE THE MODULATION.

DIVIDER SELECT

PSEUDO RANDOM

CODE GENERATOR

–

GAIN = 1

V+ – V(SET) 1.13V

fMASTER = 20MHz • 10k •

fOUT = fMASTER/2

= 20MHz • 10k/RSET

V+ – V(SET)

MASTER

OSCILLATOR

IMASTER

IREF

MDAC

CLK

2 A

VBIAS

RSET

OUT

+–

ISET =

RSET

V+ – V(SET)

SET (Pin 1/Pin 3): Frequency-Setting Resistor Input. The

value of the resistor connected between this pin and V+

determines the oscillator frequency. The voltage on this

pin is held by the LTC6908 to approximately 1.1V below

the V+ voltage. For best performance, use a precision

metal film resistor with a value between 20k and 400k

and limit the capacitance on this pin to less than 10pF.

V+ (Pin 2/Pin 1): Voltage Supply (2.7V ≤ V+ ≤ 5.5V). This

supply must be kept free from noise and ripple. It should

be bypassed directly to a ground plane with a 0.1µF

capacitor.

GND (Pin 3/Pin 2): Ground. Should be tied to a ground

plane for best performance.

OUT1 (Pin 4/Pin 6), OUT2 (Pin 5/Pin 5): Oscillator

Outputs. These pins can drive 5k and/or 10pF loads.

Larger loads may cause inaccuracies due to supply

bounce at high frequencies.

MOD (Pin 6/Pin 4): Modulation-Setting Input. This three-

state input selects among four modulation rate settings.

The MOD pin should be tied to ground for the fOUT/16

modulation rate. Floating the MOD pin selects the fOUT/32

modulation rate. The MOD pin should be tied to V+ for the

fOUT/64 modulation rate. Tying one of the outputs to the

MOD pin turns the modulation off. To detect a floating

MOD pin, the LTC6908 attempts to pull the pin toward

midsupply. This is realized with two internal current

sources, one tied to V+ and MOD and the other one tied

to ground and MOD. Therefore, driving the MOD pin high

requires sourcing approximately 2µA. Likewise, driving

the MOD pin low requires sinking 2µA. When the MOD

pin is floated, it must be bypassed by a 1nF capacitor to

ground. Any AC signal coupling to the MOD pin could

potentially be detected and stop the frequency modulation.

Exposed Pad (Pin 7/NA): Ground. The Exposed Pad must

be soldered to PCB.

(S6 Package Pin Numbers)

PIN FUNCTIONS (DCB Package/S6 Package)

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