BQ40Z50-R1: Vih fuse drive

Hello Efi,

The FUSE circuitry is described in further detail in the datasheet, section 9.2.2.2.5 FUSE Circuitry.

It also monitors the secondary protections so the gauge knows if a safety event has occurred externally.

Sincerely,

Wyatt Keller

Hi Efi,

The FUSE pin of the bq40z50-R1 is designed to ignite the chemical fuse if one of the various safety criteria is

violated. The FUSE pin also monitors the state of the secondary-voltage protection IC. Q5 ignites the chemical
fuse when its gate is high. The 7-V output of the bq294700 is divided by R16 and R6, which provides adequate
gate drive for Q5 while guarding against excessive back current into the bq294700 if the FUSE signal is high.

Best regards,

Hello Efi,

Most applications require a secondary protector for redundancy and safety. Both BQ40Z50-R1 and the secondary protector can control the blowing on the FUSE, hence BQ40Z50-R1 Fuse pin has circuitry to detect when the fuse is blown by the secondary protector. When the fuse is NOT blown the fuse pin should be high (> VIH). The BQ40Z50-R1 checks the state of the fuse pin periodically to detect whether the fuse is blown by forcing ~150nA current and checking to see if the fuse pin is > VIH. If the fuse pin is > VIH, the fuse is considered not blown by the gauge. If it's < VIH the fuse is considered blown and fuse blown flag is set. See the fuse pin internal circuit representation below and the comparator that's used to detect fuse blown.