LTC6991 datasheet(14/24 Pages) LINER

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LTC6991

6991f

applicaTions inForMaTion

Basic Operation

The simplest and most accurate method to program the

LTC6991 is to use a single resistor, RSET, between the SET

and GND pins. The design procedure is a 3-step process.

FirstselectthePOLbitsettingandNDIVvalue,thencalculate

the value for the RSET resistor.

Step 1: Select the POL Bit Setting

TheLTC6991canoperateinnormal(active-high)orinverted

(active-low) modes, depending on the setting of the POL

bit. The best choice depends on the the application.

Step 2: Select the NDIV Frequency Divider Value

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

DIVCODE which determines both the POL bit and the

NDIV value. For a given output clock period, NDIV should

be selected to be within the following range.

OUT

DIV

OUT

16 384

1 024

≤

(1)

To minimize supply current, choose the lowest NDIVvalue

(generally recommended). Alternatively, use Table 1

as a guide to select the best NDIV value for the given

application.

With POL already chosen, this completes the selection of

DIVCODE. Use Table 1 to select the proper resistor divider

or VDIV/V+ ratio to apply to the DIV pin.

Step 3: Calculate and Select RSET

The final step is to calculate the correct value for RSET

using the following equation.

SET

OUT

DIV

1 024

•

(2)

Select the standard resistor value closest to the calculated

value.

Example: Design a 1Hz oscillator with minimum power

consumption and active-high reset input.

Step 1: Select the POL Bit Setting

For noninverted (active-high) functionality, choose

POL = 0.

Step 2: Select the NDIV Frequency Divider Value

Choose an NDIV value that meets the requirements of

Equation (1), using tOUT = 1000ms:

61.04 ≤ NDIV ≤ 976.6

Potential settings for NDIV include 64 and 512. NDIV = 64

is the best choice, as it minimizes supply current by us-

ing a large RSET resistor. POL = 0 and NDIV = 64 requires

DIVCODE = 2. Using Table 1, choose R1 = 976k and

R2 = 182k values to program DIVCODE = 2.

Step 3: Select RSET

Calculate the correct value for RSET using Equation (2).

SET =

1 024

1000

763

•

Since 763k is not available as a standard 1% resistor,

substitute 768k if a –0.7% frequency shift is acceptable.

Otherwise, select a parallel or series pair of resistors such

as 576k + 187k to attain a more precise resistance.

The completed design is shown in Figure 11.

DIVCODE = 2

6991 F11

LTC6991

RST

GND

SET

RST

OUT

V+

DIV

976k

182k

RSET

763k

2.25V TO 5.5V

Figure 11. 1Hz Oscillator

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