MAX6790 Maxim Integrated Products, MAX6790 Datasheet - Page 13

• Manufacturer Part Number: MAX6790
• Description: (MAX6782 - MAX6790) Battery Monitors
• Manufacturer: Maxim Integrated Products

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DataSheet
4
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.
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
absolute resistive divider error attributable to the input
leakage current (use 0.01 for 1%), V
voltage at which LBO should activate, and I
worst-case IN_ leakage current, from the Electrical
Characteristics . For example, for 0.5% error, a 2.8V
battery minimum, and 5nA leakage, R
Calculate R
where V
Continuing the above example, and selecting V
0.5477V (10% hysteresis device), R
are other sources of error for the battery threshold,
including resistor and input monitor tolerances.
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-
U
Battery Monitors in Small TDFN and TQFN Packages
.com
Resistive Divider (MAX6782–MAX6785)
Low-Power, 1% Accurate, Dual-/Triple-/Quad-Level
INF
A
Calculating an External Hysteresis
is the fraction of the maximum acceptable
2
is the falling threshold voltage from Table 2.
using:
Applications Information
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
R
R
2
1
=
=
Resistor-Value Selection
V
e
BATT
V
A
INF
×
I
L
V
×
−
BATT
V
R
Reference Output
INF
1
BATT
2
= 681kΩ. There
1
is the battery
= 2.80MΩ.
L
is the
INF
=
old (V
(V
where e
absolute resistive divider error attributable to the input
leakage current (use 0.01 for 1%), V
ence output voltage, and I
leakage current. Calculate R
where V
calculate the percent hysteresis, use:
where V
Setting the hysteresis externally requires calculating
three resistor values, as indicated in Figure 2. First cal-
culate R
and R
where R
divider current, I
Then, determine R
where V
Finally, determine R
INR
R
) is fixed). See Table 2. Calculate R
20
20
INF
INR
HYST
1
INF
20
A
=
using:
) on the associated IN_ (the rising threshold
using:
is the fraction of the maximum acceptable
Hysteresis
V
= R
is the rising voltage.
BATT
is the desired falling voltage threshold. To
V
is the required hysteresis voltage.
TH
2
(MAX6786/MAX6787/MAX6788)
TOTAL
×
+ R
−
R
HYST
V
I
Hysteresis Resistive Divider
R
TOTAL
R
R
R
1
2
TH
R
2
HYST
4
1
HYST
using:
3
( )
, at the trip voltage using:
= R
=
=
%
(
=
as in the above example
using:
e
V
Calculating an External
e
20
V
REF
A
=
=
=
determine the total resistive-
A
INF
100
L
×
R
4
I
V
- R
TOTAL
×
I
V
1
I
V
L
is the worst-case HADJ_
HYST
using:
L
−
BATT
×
V
BATT
+
HYST
V
REF
×
R
R
INF
3
V
20
INR
V
INR
REF
−
V
3
INF
using:
is the refer-
)
13