AD8280WASTZ Analog Devices Inc, AD8280WASTZ Datasheet - Page 23

• Manufacturer Part Number: AD8280WASTZ
• Description: Lithium Ion Battery Safety Monitor
• Manufacturer: Analog Devices Inc

Specifications of AD8280WASTZ

• Function: Battery Monitor
• Battery Type: Lithium-Ion (Li-Ion)
• Voltage - Supply: 6 V ~ 30 V
• Operating Temperature: -40°C ~ 105°C
• Mounting Type: Surface Mount
• Package / Case: 48-LQFP
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

PROTECTION COMPONENTS AND PULL-UP/
PULL-DOWN RESISTORS
As shown in Figure 45, several devices are added to provide
protection in a high voltage environment. Zener Diode Z1
ensures that the six-cell stack voltage does not significantly
exceed the maximum 30 V across the part. It is recommended
that a 33 V rated Zener diode be used for Z1.
The user can also use diodes in the daisy-chain lines (anode to
cathode from higher potential to lower potential) to protect the
parts in the event that an open circuit appears on the battery
connections, causing a high reverse voltage across the AD8280
(these diodes are not shown in Figure 45). The diodes should
have a reverse voltage rating comparable to the highest voltage
of the battery system.
If diodes are used in the daisy chain, it is also recommended that
a diode be used between the top cell in the stack (anode) and
VTOP (cathode) of the top part, as well as between VBOTx
(anode) of each part and VTOP (cathode) of the next lowest
potential part in the daisy chain.
Because there are no pull-up or pull-down resistors internal to
the part, the user may want to pull down the TESTI pin of the
bottom part through a 10 kΩ resistor to VBOTx (part ground).
The addition of this resistor ensures that the part is not locked
in self-test mode if the line opens. Also, the user may want to pull
up the ENBI pin on the bottom part of a daisy chain so that if the
line opens, the chain stays in the enabled (powered up) mode.
EMI CONSIDERATIONS
To increase immunity to electromagnetic interference (EMI), use
the following components and layout schemes (see Figure 50).
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SYSTEM ACCURACY CALCULATION
When calculating system accuracy, there are four error sources
to consider:
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Use a 22 pF capacitor on each of the daisy-chain lines.
Route the daisy-chain lines on an inner PCB layer.
Use ground planes (connected to VBOTx from the higher
potential part) both over and under the daisy-chain lines to
shield them.
Route the connections from VBOTx to VTOP to best
ensure a low impedance connection between them.
Use ferrite beads on the VTOP lines as shown in Figure 50.
Use 100 nF capacitors across each of the six-cell battery stacks.
Place the AD8280 parts as close together as possible on the
board to minimize the length of the daisy-chain lines.
Trip point error (see Table 1)
Reference voltage error (see Table 1)
Resistor tolerance
Resistor temperature coefficient
Rev. A | Page 23 of 24
Sample Calculation
Following is a sample calculation for overvoltage accuracy. In
this calculation, the following conditions are assumed:
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The resulting sources of error are described in this section.
Maximum Trip Point Error
The maximum trip point error is ±15 mV.
Maximum Reference Error
The maximum reference error is as follows:
Maximum Resistor Tolerance Error
The maximum resistor tolerance error depends on the values
of the resistors. If one resistor is high and the other is low, the
worst-case error is as follows:
In this sample calculation, the maximum resistor tolerance
error is ±16 mV.
Maximum Temperature Coefficient Error
If one resistor drifts high and the other resistor drifts low, the
worst-case error is as follows:
60 kΩ + (100 ppm/°C × (25°C − (−40°C)) × 60 kΩ) = 60.39 kΩ
15 kΩ − (100 ppm/°C × (25°C − (−40°C)) × 15 kΩ) = 14.9 kΩ
(60.39/(60.39 + 14.90)) × 5.00 V = 4.010 V (error of +10 mV)
or
60 kΩ − (100 ppm/°C × (25°C − (−40°C)) × 60 kΩ) = 59.61 kΩ
15 kΩ + (100 ppm/°C × (25°C − (−40°C)) × 15 kΩ) = 15.1 kΩ
(59.61/(59.61 + 15.10)) × 5.00 V = 3.990 V (error of −10 mV)
In this sample calculation, the maximum temperature coefficient
error is ±10 mV.
Total System Accuracy
The system accuracy, or the sum of all the errors, is ±81 mV. If
the resistor pair coefficients are matched so that drift is in the
same direction, that portion of the error can be ignored, and the
total system accuracy would be ±71 mV.
Resistors used in the external resistor divider to set the trip
points are ±1%, 100 ppm/°C resistors.
Temperature range is −40°C to +85°C.
Desired overvoltage trip point is 4.0 V (resistor values
selected should be 15 kΩ and 60 kΩ).
(60/(60 + 15)) × ±50 mV = ±40 mV
(60.6/(60.6 + 14.85)) × 5.00 V = 4.016 V (error of +16 mV)
(59.4/(59.4 + 15.15)) × 5.00 V = 3.984 V (error of −16 mV)
AD8280