LTC6906 datasheet(16/28 Pages) AD

16 / 28 page

LTC6994-1/LTC6994-2

Rev. C

For more information www.analog.com

OPERATION

Changing DIVCODE After Start-Up

Following start-up, the A/D converter will continue

monitoring VDIV for changes. Changes to DIVCODE will

be recognized slowly, as the LTC6994 places a priority on

eliminating any “wandering” in the DIVCODE. The typical

delay depends on the difference between the old and

new DIVCODE settings and is proportional to the master

oscillator period.

tDIVCODE = 16 • (∆DIVCODE + 6) • tMASTER

A change in DIVCODE will not be recognized until it is

stable, and will not pass through intermediate codes. A

digital filter is used to guarantee the DIVCODE has settled

to a new value before making changes to the output. How-

ever, if the delay timing is active during the transition, the

actual delay can take on a value between the two settings.

Start-Up Time

When power is first applied, the power-on reset (POR)

circuit will initiate the start-up time, tSTART. The OUT pin

is held low during this time and the IN pin has no control

over the output. The typical value for tSTART ranges from

0.5msto8msdependingonthemasteroscillatorfrequency

(independent of NDIV):

tSTART(TYP) = 500 • tMASTER

During start-up, the DIV pin A/D converter must determine

the correct DIVCODE before the LTC6994 can respond

to an input. The start-up time may increase if the supply

or DIV pin voltages are not stable. For this reason, it is

recommended to minimize the capacitance on the DIV

pin so it will properly track V+. Less than 100pF will not

extend the start-up time.

At the end of tSTART the DIVCODE and IN pin settings are

recognized, and the state of the IN pin is transferred to the

output (without additional delay). If IN is high at the end of

tSTART, OUT will go high. Otherwise OUT will remain low.

The LTC6994-2 with POL = 1 is the exception because it

inverts the signal. At this point, the LTC6994 is ready to

respond to rising/falling edges on the input.

DIV

500mV/DIV

2V/DIV

OUT

2V/DIV

LTC6994-1

V+ = 3.3V

RSET = 200k

500µs/DIV

699412 F07a

512µs

256µs

4µs

Figure 7a. DIVCODE Change from 0 to 2

DIV

500mV/DIV

2V/DIV

OUT

2V/DIV

LTC6994-1

V+ = 3.3V

RSET = 200k

500µs/DIV

699412 F07b

256µs

4µs

512µs

Figure 7b. DIVCODE Change from 2 to 0

V+

OUT

tSTART

(IN IGNORED)

tPD

699412 F08

IF IN = 1 AT END OF tSTART*

IF IN = 0 AT END OF tSTART*

*LTC6994-2 WITH POL = 1 INVERTS THE OUTPUT

Figure 8. Start-Up Timing Diagram

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