LTC1998CS6 datasheet(10/8 Pages) LINER

10 / 8 page

LTC1998

1998fa

APPLICATIONS INFORMATION

The calculation of the resistor values R1, R2 and R3 is

quite straightforward and similar to the procedure outlined

in the previous paragraph.

Choose a value for the sum of R1 + R2 + R3 as well as the

values for low battery threshold and hysteresis.

Solve for resistor R1:

VTH

=+ +

⎛

⎝⎜

⎞

⎠⎟

()

–

Solve for the sum of

() (

)

–

VBATT Th

+ +

⎛

⎝⎜

⎞

⎠⎟

then solve for R2 and R3.

Example: A system needs to detect a low battery

voltage of 3V (VBATT.Th = 3V) with 250mV hysteresis

(VTH2 = 3.25V). Set the value of the resistor divider

(R1 + R2 + R3) = 1M.

R1 = 539k, R1 + R2 = 667k, R2 = 128k, R3 = 333k.

Choose the closest 1% values, that is 536k, 332k, 127k.

Figure 4 graphically shows the function of the LTC1998 as

described above.

Figure 4. LTC1998 Function Plot

VERSATILE OUTPUT DRIVER

VLOGIC, BATTLO (Pins 5,6)

The LTC1998 uses a CMOS push-pull output stage to drive

the low battery output signal. This output pin (BATTLO)

has a separate supply pin, (VLOGIC) that can be used to

provide an output voltage rail matching the VDD logic of

microprocessors. The VLOGIC pin may be tied to a volt-

age lower than the voltage at the BATT pin. The VLOGIC

pin may also be tied to a voltage higher than VBATT via a

series resistor greater than 10k. The output will then act

as an open-drain device.

In a given application, if it is possible for BATTLO to be

shorted to GND or a supply, a series resistor should be

added to limit the short-circuit current to 5mA.

1998 F04

PROGRAMMABLE

THRESHOLD RANGE

HYSTERESIS

PROGRAMMED

HYSTERESIS

THRESHOLD

PROGRAMMED

LOW BATT

THRESHOLD

VLOGIC

BATTLO

BATTERY VOLTAGE

RECOVERS UNDER

REDUCED LOAD

BATTERY

VOLTAGE

3.25V

2.50V

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