MAX6777LT+T Maxim Integrated Products, MAX6777LT+T Datasheet - Page 7

• Manufacturer Part Number: MAX6777LT+T
• Description: IC MONITOR BAT LP 6-UDFN
• Manufacturer: Maxim Integrated Products

Specifications of MAX6777LT+T

• Function: Battery Monitor
• Battery Type: Alkaline, Li-Ion, NiCd, NiMH
• Voltage - Supply: 1 V ~ 5.5 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 6-WFDFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Other names: MAX6777LT+TTR

Table 2. Typical Falling and Rising Thresholds for
MAX6775/MAX6776/MAX6779/MAX6780/MAX6781
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.
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.
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.
Figure 4. Hysteresis
DEVICE OPTION
LBO
V
BATT
V
V
LBIR
LBIF
C
A
B
_______________________________________________________________________________________
t
PD
HYSTERESIS (%)
PERCENT
Monitors in µDFN and SC70 Packages
MAX6775
MAX6776
V
HYST
0.5
10
5
Low-Power, 1%-Accurate Battery
Fixed Hysteresis
FALLING THRESHOLD
Hysteresis
t
PD
(V
1.2159
1.1609
1.0998
LBIF
) (V)
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.
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
where e
divider error (use 0.01 for 1%), V
age at which LBO should activate, and I
case LBI leakage current. For example, with 0.5%
accuracy, a 2.8V battery minimum, and 5nA leakage,
R
Calculate R
where V
Continuing the above example, select V
(10% hysteresis device) and R
H
= 2.80MΩ.
RISING THRESHOLD
A
LBIF
is the maximum acceptable absolute resistive
L
(V
is the falling threshold voltage from Table 2.
using:
LBIR
1.222
1.222
1.222
R
Applications Information
R
L
H
) (V)
=
=
−
Resistor-Value Selection
V
e
V
LBIF
A
LBIF
x V
I
−
L
L
V
x R
BATT
HYSTERESIS VOLTAGE
BATT
Adjustable Hysteresis
= 1.81MΩ.
BATT
H
(V
is the battery volt-
HYST
LBIF
6.11
61.1
L
122
is the worst-
) (mV)
= 1.0998V
H
using:
7