BQ76PL455APFCT datasheet(97/125 Pages) TI1

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bq76PL455A

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SLUSCL0A – SEPTEMBER 2016 – REVISED OCTOBER 2016

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Application Information (continued)

Select zener diodes that meet the following conditions:

1. The bq76PL455 inputs are protected from input voltage transients greater than 5.5 V. Any transients greater

than 5.5 V must be clamped to less than 6.5 V for an accumulative time duration not to exceed 0.1% of the

device 10-year lifetime.

2. The zener maximum reverse current (Iz) at normal battery-cell voltage levels are as low as possible to keep

the quiescent system-current draw low. Additionally, any leakage through the zener diode affects the total

channel accuracy measurements as it adds to the voltage drop across the filter resistor.

3. The zener must be capable of withstanding instantaneous or continuous currents that the bq76PL455A

experiences during fault events. These events can include cable connect/disconnect, inrush, or reverse

battery voltage. Depending on the fault and the direction of current flow, the maximum power dissipation for

the zener diode must not be exceeded to stay within maximum operating conditions.

Additionally, a series resistor and bypass capacitor are required for each of the VSENSE inputs. The series

resistor (R132 and R138 in Figure 28) serves two functions:

1. They protect the AFE inputs from in-rush currents during hot plug-in. This requirement limits the input series

R to a minimum of 100 Ω. Keep this resistance as low as possible to minimize input voltage offset, which is

also subject to drift over temperature. These requirements limit the maximum value to 1 kΩ. Error due to

input bias currents, on the front end of the AFE, is directly proportional to the value of the resistors. The

voltage measurement error can be calculated as:

voltage measurement error = 2 × R × ISENSE

(10)

2. The resistor and capacitor (R132 and C61 in Figure 28) provide an RC filter for high-frequency noise on the

AFE inputs. The tuning of this filter cutoff is adjustable and depends on the expected frequency of noise in

the system. Select a minimum 0.1-µF capacitor connected from the VSENSE input to the battery pack GND

(BAT0 in Figure 28). Calculate the cutoff frequency using Equation 11:

(11)

3. Ensure that the RC-filter components are as close to the device as possible. The capacitor location is

extremely important and must be given priority for optimum performance.

4. For applications that require a very low filter cutoff frequency, connect a differential capacitor between the

VSENSE lines to provide the bulk of the AFE input filtering. The bias voltage on these differential capacitors

are the same (or very close), therefore, anti-aliasing is improved.

Additional components are necessary to improve EMC performance of bq76PL455A in applications with

electrically noisy environments.

•

Use ferrite beads or small inductors in series with the cell inputs between the cell output and the series

resistor to the VSENSE input. The bead and small capacitor must be located near each other.

•

Add a 0.0033-µF capacitor from each cell input to the battery pack (BAT0 in Figure 29). Adjust the value of

capacitance to satisfy the PCB layout and field conditions for the application.

BQ76PL455APFCT Datasheet(PDF) 97 Page - Texas Instruments

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