LTC6900CS5 datasheet(9/12 Pages) LINER

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LTC6900

6900f

APPLICATIO S I FOR ATIO

V+

RSET1

RSET2

V+

6900 F07

GND

LTC6900

3V OR 5V

SET

OUT

DIV

÷10

÷100

÷1

fOSC = 10MHz •

()

20k

N • RSET1

fOSC = 10MHz •

()

20k

N • RSET1//RSET2

Figure 7

A Ground Referenced Voltage Controlled Oscillator

The LTC6900 output frequency can also be programmed

by steering current in or out of the SET pin, as conceptually

shown in Figure 8. This technique can degrade accuracy as

the ratio of (V+ – VSET) / IRES is no longer uniquely

dependent of the value of RSET, as shown in the LTC6900

Block Diagram. This loss of accuracy will become notice-

able when the magnitude of IPROG is comparable to IRES.

The frequency variation of the LTC6900 is still monotonic.

Figure 9 shows how to implement the concept shown in

Figure 8 by connecting a second resistor, RIN, between the

SET pin and a ground referenced voltage source, VIN.

For a given power supply voltage in Figure 9, the output

frequency of the LTC6900 is a function of VIN, RIN, RSET

and (V+ – VSET) = VRES:

MHz

OSC

SET

RES

SET

−

()

























••

•

(1)

When VIN = V+, the output frequency of the LTC6900

assumes the highest value and it is set by the parallel

combination of RIN and RSET. Also note, the output fre-

quency, fOSC, is independent of the value of VRES = (V+ –

VSET) so the accuracy of fOSC is within the data sheet limits.

When VIN is less than V+, and expecially when VIN ap-

proaches the ground potential, the oscillator frequency,

fOSC, assumes its lowest value and its accuracy is affected

by the change of VRES = (V+ – VSET). At 25°C VRES varies

±8%, assuming the variation of V+ is ±5%. The tem-

perature coefficient of VRES is 0.02%/°C.

By manipulating the algebraic relation for fOSC above, a

simple algorithm can be derived to set the values of

external resistors RSET and RIN, as shown in Figure 9.

1. Choose the desired value of the maximum oscillator

frequency, fOSC(MAX), occurring at maximum input volt-

age VIN(MAX) ≤ V+.

2. Set the desired value of the minimum oscillator fre-

quency, fOSC(MIN), occurring at minimum input voltage

VIN(MIN) ≥ 0.

3. Choose VRES = 1.1 and calculate the ratio of RIN/RSET

from the following:

SET

IN MAX

OSC MAX

OSC MIN

IN MIN

RES

OSC MAX

OSC MIN

−

()−







−

()

−













−

()

1 (2)

Figure 9. Implementation of Concept Shown in Figure 8

Figure 8. Concept for Programming via Current Steering

V+

RSET

IPR

V+

6900 F08

GND

LTC6900

0.1

µF

OPEN

SET

OUT

DIV

÷10

÷100

÷1

IRES

V+

RSET

VRES

RIN

VIN

V+

6900 F09

GND

LTC6900

0.1

µF

fOSC

OPEN

SET

OUT

DIV

÷10

÷100

÷1

–

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