ADC121C021CIMKX NSC [National Semiconductor], ADC121C021CIMKX Datasheet - Page 28

• Manufacturer Part Number: ADC121C021CIMKX
• Description: I2C-Compatible, 12-Bit Analog-to-Digital Converter (ADC) with Alert Function
• Manufacturer: NSC [National Semiconductor]

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2.3 INTELLIGENT BATTERY MONITOR
The ADC121C021 is easily used as an intelligent battery
monitor. The simple circuit shown in Figure 20, uses the
ADC121C021, the LP2980 fixed reference, and a resistor di-
vider to implement an intelligent battery monitor with a window
supervisory feature. The window supervisory feature is im-
plemented by the "out of range" alert function. When the
battery is recharging, the Over Range Alert will indicate that
the charging cycle is complete (see Figure 21). When the
battery is nearing depletion, the Under Range Alert will indi-
cate that the battery is low (see Figure 22).
FIGURE 20. Intelligent Battery Monitor Circuit
FIGURE 22. Discharge Cycle
FIGURE 21. Recharge Cycle
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In addition to the window supervisory feature, the
ADC121C021 will allow the controller to read the battery volt-
age at any time during operation. Reading the conversion
result via the I
ing.
The accurate voltage reading and the alert feature will allow
a controller to improve the efficiency of a battery-powered
device. During the discharge cycle, the controller can switch
to a low-battery mode, safely suspend operation, or report a
precise battery level to the user. During the recharge cycle,
the controller can implement an intelligent recharge cycle,
decreasing the charge rate when the battery charge nears
capacity.
2.3.1 Trickle Charge Controller
While a battery is discharging, the ADC121C021 can be used
to control a trickle charge to keep the battery near full capacity
(see Figure 23). When the alert output is active, the battery
will recharge. An intelligent recharge cycle will prevent over-
charging and damaging the battery. With a trickle charge, the
battery powered device can be disconnected from the charger
at any time with a full charge.
2.4 LAYOUT, GROUNDING, AND BYPASSING
For best accuracy and minimum noise, the printed circuit
board containing the ADC121C021 should have separate
analog and digital areas. The areas are defined by the loca-
tions of the analog and digital power planes. Both of these
planes should be located on the same board layer. There
should be a single ground plane. A single, solid ground plane
is preferred if digital return current does not flow through the
analog ground area. Frequently a single ground plane design
will utilize a "fencing" technique to prevent the mixing of ana-
log and digital ground current. Separate ground planes should
only be utilized when the fencing technique is inadequate.
The separate ground planes must be connected in one place,
preferably near the ADC121C021. Special care is required to
guarantee that signals do not pass over power plane bound-
aries. They must always have a continuous return path below
their traces.
The ADC121C021 power supply should be bypassed with a
4.7µF and a 0.1µF capacitor as close as possible to the device
with the 0.1µF right at the device supply pin. The 4.7µF ca-
pacitor should be a tantalum type and the 0.1µF capacitor
should be a low ESL type. The power supply for the AD-
C121C021 should only be used for analog circuits.
Avoid crossover of analog and digital signals and keep the
clock and data lines on the component side of the board. The
clock and data lines should have controlled impedances.
2
C interface provides an accurate voltage read-
FIGURE 23. Trickle Charge
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