LTC6906 datasheet(10/30 Pages) AD

10 / 30 page

LTC6990

Rev. D

For more information www.analog.com

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 =

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:

fMASTER =

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 =

tOUT

1MHz • 50k

NDIV

•

ISET

VSET

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

fOUT =

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.

Figure 1. Simple Technique for Setting DIVCODE

6990 F01

LTC6990

V+

DIV

GND

2.25V TO 5.5V

LTC6906 Datasheet(PDF) 10 Page - Analog Devices

LTC6906 Datasheet(HTML) 10 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Datasheet Download

Link URL