LTC6990 查看數據表（PDF） - Analog Devices

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LTC6990

TimerBlox: Voltage Controlled Silicon Oscillator

Analog Devices 

LTC6990

OPERATION

The LTC6990 is built around a master oscillator with a

1MHz maximum frequency. The oscillator is controlled

by the SET pin current (ISET) and voltage (VSET), with a

1MHz • 50k conversion factor that is accurate to ±0.8%

under typical conditions.

f MASTER =

1

tMASTER

= 1MHz

•

50k

•

ISET

VSET

A feedback loop maintains VSET at 1V ±30mV, leaving ISET

as the primary means of controlling the output frequency.

The simplest way to generate ISET is to connect a resistor

(RSET) between SET and GND, such that ISET = VSET/RSET.

The master oscillator equation reduces to:

f MASTER

=

1

tMASTER

=

1MHz • 50k

RSET

From this equation it is clear that VSET drift will not affect

the output frequency when using a single program resis-

tor (RSET). Error sources are limited to RSET tolerance and

the inherent frequency accuracy ∆fOUT of the LTC6990.

RSET values between 50k and 800k (equivalent to ISET

between 1.25µA and 20µA) produce the best results,

although RSET may be reduced to 25k (ISET = 40µA) with

reduced accuracy.

The LTC6990 includes a programmable frequency divider

which can further divide the frequency by 1, 2, 4, 8, 16,

32, 64 or 128 before driving the OUT pin. The divider ratio

NDIV is set by a resistor divider attached to the DIV pin.

fOUT

=

1

tOUT

=

1MHz • 50k

NDIV

•

ISET

VSET

With RSET in place of VSET/ISET the equation reduces to:

fOUT

=

1

tOUT

=

1MHz • 50k

NDIV • RSET

DIVCODE

The DIV pin connects to an internal, V+ referenced 4-bit

A/D converter that monitors the DIV pin voltage (VDIV) to

determine the DIVCODE value. DIVCODE programs two

settings on the LTC6990:

1. DIVCODE determines the output frequency divider set-

ting, NDIV.

2. DIVCODE determines the state of the output when dis-

abled, via the Hi-Z bit.

VDIV may be generated by a resistor divider between V+

and GND as shown in Figure 1.

2.25V TO 5.5V

V+

LTC6990

R1

DIV

R2

GND

6990 F01

Figure 1. Simple Technique for Setting DIVCODE

Rev. D

10

For more information www.analog.com

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