LTC6905CS5 datasheet(5/8 Pages) LINER

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LTC6905

6905fa

BLOCK DIAGRA

THEORY OF OPERATIO

As shown in the Block Diagram, the LTC6905’s master

oscillator is controlled by the ratio of the voltage between

the V+ and SET pins and the current entering the SET pin

(IRES). The voltage on the SET pin is forced to approxi-

mately 1V below V+ by the PMOS transistor and its gate

bias voltage.

A resistor RSET, connected between the V+ and SET pins,

“locks together” the voltage (V+ – VSET) and current, IRES,

variation. This provides the LTC6905’s high precision. The

master oscillation frequency reduces to:

MHz

SET

Ω

168 5

.•

To extend the output frequency range, the master oscilla-

tor signal is divided by 1, 2 or 4 before driving OUT (Pin

5). The LTC6905 is optimized for use with resistors

between 10k and 25k, corresponding to oscillator fre-

quencies between 17.225MHz and 170MHz. The divide-

by value is determined by the state of the DIV input

(Pin 4). Tie DIV to V+ or drive it to within 0.4V of V+ to

select ÷1. This is the highest frequency range, with the

Figure 1. RSET vs Output Frequency

master output frequency passed directly to OUT. The DIV

pin may be floated or driven to midsupply to select ÷2, the

intermediate frequency range. The lowest frequency range,

÷4, is selected by tying DIV to GND or driving it below

0.5V. Figure 1 shows the relationship between RSET,

divider setting and output frequency, including the over-

lapping frequencies.

OUTPUT FREQUENCY (MHz)

÷4 ÷2

÷1

6905 F01

110

160

–

GAIN = 1

V+

VBIAS

IRES

RSET

SET

GND

MASTER OSCILLATOR

PROGRAMMABLE

DIVIDER

(÷1, 2 OR 4)

VRES = 1V ±5%

(V+ – VSET)

THREE-STATE

INPUT DETECT

GND

V+

15µA

6905 BD

15µA

OUT

DIVIDER

SELECT

DIV

fOSC =

fMO

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