Applications information – Rainbow Electronics MAX6781 User Manual
Page 7
MAX6775–MAX6781
Low-Power, 1%-Accurate Battery
Monitor in µDFN and SC70 Packages
_______________________________________________________________________________________
7
The MAX6779, MAX6780, and MAX6781 monitor two
battery levels or two independent voltages. A common
application for this type of dual-battery monitor is to use
one output as an early warning signal and the other as
a dead-battery indicator.
Hysteresis
Input hysteresis defines two thresholds, separated by a
small voltage (the hysteresis voltage), configured so
the output asserts when the input falls below the falling
threshold, and deasserts only when the input rises above
the rising threshold. Figure 4 shows this graphically.
Hysteresis removes, or greatly reduces, the possibility of
the output changing state in response to noise or battery
terminal voltage recovery after load removal.
Fixed Hysteresis
The MAX6775/MAX6776/MAX6779/MAX6780/MAX6781
have factory-set hysteresis for ease of use, and reduce
component count. For these devices, the absolute hys-
teresis voltage is a percentage of the internally generat-
ed reference. The amount depends on the device
option. “A” devices have 0.5% hysteresis, “B” devices
have 5% hysteresis, and “C” devices have 10% hystere-
sis. Table 2 presents the threshold voltages for devices
with internally fixed hysteresis.
Adjustable Hysteresis
The MAX6777/MAX6778 offer external hysteresis con-
trol through the resistive divider that monitors battery
voltage. Figure 3 shows the connections for external
hysteresis. See the Calculating an External Hysteresis
Resistive Divider section for more information.
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. Figure 1 calculates the optimum value for R
H
using:
where e
A
is the maximum acceptable absolute resistive
divider error (use 0.01 for 1%), V
BATT
is the battery volt-
age at which LBO should activate, and I
L
is the worst-
case LBI leakage current. For example, with 0.5%
accuracy, a 2.8V battery minimum, and 5nA leakage,
R
H
= 2.80M
Ω.
Calculate R
L
using:
where V
LBIF
is the falling threshold voltage from Table 2.
Continuing the above example, select V
LBIF
= 1.0998V
(10% hysteresis device) and R
L
= 1.81M
Ω.
R
V
x R
V
V
L
LBIF
H
LBIF
BATT
=
−
−
R
e
x V
I
H
A
BATT
L
=
V
BATT
LBO
V
LBIR
V
LBIF
V
HYST
t
PD
t
PD
MAX6775
MAX6776
Figure 4. Hysteresis
Table 2. Typical Falling and Rising Thresholds for
MAX6775/MAX6776/MAX6779/MAX6780/MAX6781
DEVICE OPTION
PERCENT
HYSTERESIS (%)
FALLING THRESHOLD
(V
LBIF
) (V)
RISING THRESHOLD
(V
LBIR
) (V)
HYSTERESIS VOLTAGE
(V
HYST
) (mV)
A
0.5
1.2159
1.222
6.11
B
5
1.1609
1.222
61.1
C
10
1.0998
1.222
122