LTC6991HS6 View Datasheet(PDF) - Analog Devices

Part Name

Description

Manufacturer

LTC6991HS6

TimerBlox: Resettable, Low Frequency Oscillator

Analog Devices 

LTC6991

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.

First select the POL bit setting and NDIV value, then cal-

culate the value for the RSET resistor.

Alternatively, Linear Technology offers the easy to use

TimerBlox Designer tool to quickly design any LTC6991

based circuit. Download the free TimerBlox Designer soft-

ware at www.linear.com/timerblox.

Step 1: Select the POL Bit Setting

The LTC6991 can operate in normal (active-high) or

inverted (active-low) modes, depending on the set-

ting 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.

tOUT

16.384ms

≤

NDIV

≤

tOUT

1.024ms

(1)

To minimize supply current, choose the lowest NDIV value

(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.

RSET

=

50k

1.024ms

•

tOUT

NDIV

(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 using 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).

RSET

=

50k

1.024ms

•

1 0 0 0m s

64

=

763k

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.

RST RST

OUT

LTC6991

2.25V TO 5.5V

GND

V+

SET

DIV

RSET

763k

R1

976k

DIVCODE = 2

R2

182k

6991 F11

Figure 11. 1Hz Oscillator

6991fc

14

For more information www.linear.com/LTC6991

Share Link: