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 e
A
is the fraction of the maximum acceptable
absolute resistive divider error attributable to the input
leakage current (use 0.01 for 1%), V
BATT
is the battery
voltage at which LBO should activate, and I
L
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, R
1
= 2.80MΩ.
Calculate R
2
using:
where V
INF
is the falling threshold voltage from Table 2.
Continuing the above example, and selecting V
INF
=
0.5477V (10% hysteresis device), R
2
= 681kΩ. 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 (V
INF
) on the associated IN_ (the rising threshold
(V
INR
) is fixed). See Table 2. Calculate R
3
using:
where e
A
is the fraction of the maximum acceptable
absolute resistive divider error attributable to the input
leakage current (use 0.01 for 1%), V
REF
is the refer-
ence output voltage, and I
L
is the worst-case HADJ_
leakage current. Calculate R
4
using:
where V
INF
is the desired falling voltage threshold. To
calculate the percent hysteresis, use:
where V
INR
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 R
1
using:
and R
20
using:
where R
20
= R
2
+ R
HYST
determine the total resistive-
divider current, I
TOTAL
, at the trip voltage using:
Then, determine R
HYST
using:
where V
HYST
is the required hysteresis voltage.
Finally, determine R
2
using:
R
2
= R
20
- R
HYST
R
V
I
HYST
HYST
TOTAL
=
