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AD8280

Rev. C | Page 23 of 24

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).

•

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.

SYSTEM ACCURACY CALCULATION

When calculating system accuracy, there are four error sources

to consider:

•

Trip point error (see Table 1)

•

Reference voltage error (see Table 1)

•

Resistor tolerance

•

Resistor temperature coefficient

Sample Calculation

Following is a sample calculation for overvoltage accuracy. In

this calculation, the following conditions are assumed:

•

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).

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:

(60/(60 + 15)) × ±50 mV = ±40 mV

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:

(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)

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)

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.

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