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bq24170 Optional Diodes in Application Schematic

Keith Keller
Expert 6260 points
Other Parts Discussed in Thread: BQ24170

Hello,

I am designing with the bq24170 charger.

In the datasheet there are 4 different typical applications schematics.
Please see < attached > WORD document

Figure 3 shows:
  A diode from SW pin to ground.
  A diode from BTST pin to REGN.

Figure 4 shows:
  D1 optional diode, but not the diode from SW to Ground.

Could you please explain:
1. why we would use optional diode D1?
2. why there is a diode shown from SW to Ground on figures 1, 2, 3, but not 4?
3. what are example part numbers you would recommend?

Thanks very much, Keith

bq24170 scheamtic questions.docx

  • 0
    TI__Expert 3160 points

    Keith,

     

    The optional diode improves efficiency, but drives up the BOM cost so this is why we list it as optional.

    The diode from SW to GND is required in most cases.  In Fig 4, the application is configured for a very low charge current (<900mA) and so can be omitted.

    You can reference the bq24170 EVM  for example part numbers and layout.

     

    Thanks,

    Rachel

  • 0 in reply to Rachel Moore
    TI__Expert 6260 points
    Hi Rachel,

    Thank you.
    I see D3 called out as B220A on the evaluation kit.
    I understand now this is the synchronous rectifying schottky diode from switch-node to ground for better efficiency at higher output current.

    However I do not see the diode from BTST to REGN on the evaluation module.
    Is this a Zener diode to protect BTST from over-voltage condition?
    Or is this again another schottky diode to improve efficiency of the IC’s gate drive boost converter?

    Thank you, Keith
  • 0 in reply to Keith Keller
    TI__Expert 6260 points

    Hi Rachel,

    Additionally D2 on the evaluation kit is only rated to 2A of current. (B220A-13-F, Diode, Schottky, 2A, 20V, SMA)

    Is this sufficient for the bq24170 which is 4A device?

    Thank you, Keith

  • 0 in reply to Keith Keller
    Intellectual 420 points

    Keith,

    Did you ever learn how to properly spec the diodes?  I'm coming to the same questions as you.

    How to specify the diode for GND to SW? I thought this was a synchronous rectifier with a low-side n-channel FET?  Is the diode here to assist and improve efficiency? Or is the low side FET not designed to drive the full 4A?

    As for the BST diode, what's the nature of this?  Paralleling diodes (optional diode + internal diode) usually don't work that well.

  • 0 in reply to Kyle Manna
    Intellectual 420 points

    Found another eval board called "SMP-BQ24170-MVK"

    Same 2A diode but claims the board can be set to 2.5A.

    www.ti.com/.../slvu695.pdf

  • 0 in reply to Kyle Manna
    TI__Expert 6260 points

    Hello Kyle,

    The diode from GND to SW is to improve efficiency and reduce heat in the charger IC.
    This diode is only conducting current for a very short time: after high-side FET turns off and before low-side FET turns on.
    You want to use a schottky type diode.

    You need to use a diode with a current rating equal to, or greater than, your desired output current.
    The voltage rating needs to be equal to, or greater than, your Input Voltage.
    However it's good practice to derate components by 20% or more, so you would want current and voltage greater than I_out and V_in.

    Best regards, Keith

  • 0 in reply to Keith Keller
    Intellectual 420 points

    Hi Keith,

    Thanks for the clarification, I appreciate it.