LTC6906 datasheet(15/30 Pages) AD

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LTC6990

Rev. D

For more information www.analog.com

APPLICATIONS INFORMATION

Start-Up Behavior

When first powered up, the output is high impedance. If

the output is enabled (OE = 1) at the end of the start-up

time, the output will go low for one tMASTER cycle (or half

a tOUT cycle if NDIV < 4) before the first rising edge. If the

output is disabled (OE = 0) at the end of the start-up time,

the output will drop to a low output if the Hi-Z bit = 0, or

simply remain floating if Hi-Z = 1.

Basic Fixed Frequency Operation

The simplest and most accurate method to program the

LTC6990 for fixed frequency operation is to use a sin-

gle resistor, RSET, between the SET and GND pins. The

design procedure is a simple two step process. First select

the NDIV value and then calculate the value for the RSET

resistor.

Alternatively, Analog Devices offers the easy to use

TimerBlox Designer tool to quickly design any LTC6990

based circuit. Use the free TimerBlox LTC6990: Voltage

Controlled Oscillator Web-Based Design Tool.

Step 1: Selecting the NDIV Frequency Divider Value

As explained earlier, the voltage on the DIV pin sets the

DIVCODE which determines both the Hi-Z bit and the

NDIV value. For a given output frequency, NDIV should be

selected to be within the following range.

62.5kHz

fOUT

≤ NDIV ≤

1MHz

fOUT

(1a)

To minimize supply current, choose the lowest NDIV value

(generally recommended). For faster start-up or decreased

jitter, choose a higher NDIV setting. Alternatively, use

Table 1 as a guide to select the best NDIV value for the

given application. After choosing the value for NDIV, use

Table 1 to select the proper resistor divider or VDIV/V+

ratio to apply to the DIV pin.

Step 2: Calculate and Select RSET

The final step is to calculate the correct value for RSET

using the following equation.

RSET =

1MHz • 50k

NDIV • fOUT

(1b)

Select the standard resistor value closest to the calculated

value.

Example: Design a 20kHz Oscillator with Minimum

Power Consumption

Step 1: Selecting the NDIV Frequency Divider Value

First, choose an NDIV value that meets the requirements

of Equation (1a).

3.125 ≤ NDIV ≤ 50

Potential settings for NDIV include 4, 8, 16, and 32. NDIV

= 4 is the best choice, as it minimizes supply current by

using a large RSET resistor. Using Table 1, choose the R1

and R2 values to program DIVCODE to either 2 or 13,

depending on the desired behavior when the output is

disabled.

Step 2: Select RSET

Calculate the correct value for RSET using Equation (1b).

RSET =

1MHz • 50k

4 • 20kHz

= 625k

Since 625k is not available as a standard 1% resistor,

substitute 619k if a 0.97% frequency shift is acceptable.

Otherwise, select a parallel or series pair of resistors such

as 309k and 316k to attain a more precise resistance.

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