MAX6783 datasheet(13/17 Pages) MAXIM

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Reference Output

The reference output can provide up to 1mA of output

current. The output is not buffered. Excessive loading

affects the accuracy of the thresholds. An external

capacitor is not required for stability and is stable for

capacitive loads up to 50pF. In applications where the

load or the supply can experience step changes, a

capacitor reduces the amount of overshoot (under-

shoot) and improves the circuit’s transient response.

Place the capacitor as close to the device as possible

for best performance.

Applications Information

Resistor-Value Selection

Choosing the proper external resistors is a balance

between accuracy and power use. The input to the volt-

age monitor, while high impedance, draws a small cur-

rent, and that current travels through the resistive

divider, introducing error. If extremely high resistor val-

ues are used, this current introduces significant error.

With extremely low resistor values, the error becomes

negligible, but the resistive divider draws more power

from the battery than necessary, and shortens battery

life. See Figure 6 and calculate the optimum value for

R1 using:

where eA is the fraction of the maximum acceptable

absolute resistive divider error attributable to the input

leakage current (use 0.01 for 1%), VBATT is the battery

voltage at which LBO should activate, and IL is the

worst-case IN_ leakage current, from the

Electrical

Characteristics. For example, for 0.5% error, a 2.8V

battery minimum, and 5nA leakage, R1 = 2.80M

Ω.

Calculate R2 using:

where VINF is the falling threshold voltage from Table 2.

Continuing the above example, and selecting VINF =

0.5477V (10% hysteresis device), R2 = 681k

Ω. There

are other sources of error for the battery threshold,

including resistor and input monitor tolerances.

Calculating an External Hysteresis

Resistive Divider (MAX6782–MAX6785)

To set the hysteresis, place a resistive divider from REF

to HADJ_ as shown in Figure 6. The resistive divider

sets voltage on HADJ_, which controls the falling thresh-

old (VINF) on the associated IN_ (the rising threshold

(VINR) is fixed). See Table 2. Calculate R3 using:

where eA is the fraction of the maximum acceptable

absolute resistive divider error attributable to the input

leakage current (use 0.01 for 1%), VREF is the refer-

ence output voltage, and IL is the worst-case HADJ_

leakage current. Calculate R4 using:

where VINF is the desired falling voltage threshold. To

calculate the percent hysteresis, use:

where VINR is the rising voltage.

Calculating an External

Hysteresis Resistive Divider

(MAX6786/MAX6787/MAX6788)

Setting the hysteresis externally requires calculating

three resistor values, as indicated in Figure 2. First cal-

culate R1 using:

and R20 using:

where R20 = R2 + RHYST determine the total resistive-

divider current, ITOTAL, at the trip voltage using:

Then, determine RHYST using:

where VHYST is the required hysteresis voltage.

Finally, determine R2 using:

R2 = R20 - RHYST

HYST

TOTAL

BATT

120

as in the above example

BATT

−

(

)

BATT

1 =

Hysteresis

INR

INF

INR

() =×

−

100

INF

REF

INF

−

REF

3 =

INF

BATT

INF

−

BATT

1 =

Low-Power, 1% Accurate, Dual-/Triple-/Quad-Level

Battery Monitors in Small TDFN and TQFN Packages

______________________________________________________________________________________

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