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BQ76920EVM: Questions about BOOT and protection circuit

Paul K52
Intellectual 640 points
Part Number: BQ76920EVM

Hello, 

Question 1:

Refering to the BQ76920EVM on page 36, you can see a schematic circuit about the BQ7692000PW. My question is about the components: D1, D6, C17 and C18. What is the purpose of them?

Do they protect the circuit about overvoltage? Would it be okay, if I would not use this part of the circuit?

Question 2:

The TS1 Pin of the BQ7692000PW has two fuctions: It should measure the voltage and with this pin we are able to boot up the monitor-IC. Why are you using a 3k resistor at R22 ( Refering to the BQ76920EVM on page 36)

Is there a reason why you are using the voltage of the cell 1 "C1" to provide the TS1 with power?

  • 0
    TI__Guru 54715 points
    Hi Paul,
    1: D1 is a TVS on the pack it prevents large transients, If you happen to connect a current limited power supply to the EVM in reverse, it may save the board from damage. You may not need transient suppression in your battery design. Some designers do not like a TVS across the battery because it is a single point of failure and if they need transient suppression choose some other technique. A fuse between the cells and board may be desired.
    D6 is a flyback or free-wheel diode, it will clamp the excursion of PACK- to PACK+ on a protection event with inductive load. You may or may not need this in your battery design.
    C17 and C18 are for ESD attenuation. Two caps are used so if one shorts the other still blocks DC current. You will likely want some ESD protection in your design.
    While discussing this circuit, you may have seen the forum posts about the E1 spark gap, this is a single point contamination risk which would bypass the proteciton FETs and is not recommended for a battery design.

    2. TS1 needs a rising edge to boot the part. C1 is a convenient voltage source. Since 1 V is needed, at room temperature with a thermistor mounted the circuit will need a cell voltage of 13/10 = 1.3 V to boot. The resistor prevents direct connection of the cell to the IC pin. Note that the cell voltage could be greater than the abs max of the TS pin, but this is a concern for operation rather than the instant of boot. A divider is desired, select values to work over the desired temperature range when using a thermistor. Other signal sources can be used for boot, see the apnote on the topic if you need ideas.