BQ76940EVM - Why Schottky diode are only present from 6th Cell to 15th Cell and not in first 5 Cells

Girish Gangadharaiah

Other Parts Discussed in Thread: BQ76940EVM, BQ76940

Hey Everyone,

I wanted to use BQ7940 in my design, so want to understand few things, So I observed in SCHEMATICs of BQ76940EVM that Schottky diode (D32-D37 & D16-D21) are only present from 6th Cell to 15th Cell and not in first 5 Cells.

Please find attached the AFE schematic below, Please help me understand the same.

I am not able to attach the SCHEMATIC, It is available at " http://www.ti.com/lit/ug/slvu925b/slvu925b.pdf "

over 8 years ago

0 Vish Nadarajah over 8 years ago

TI__Expert 6085 points

its due to the architecture of the bq76940. The device has three groups. On the lower group the inputs do not go below VSS. On the upper groups, the power filter capacitors hold the group reference voltage above battery - and current flows from the input resistors. after the input caps are depleted , current will flow from the device inputs continuing the discharge of the lower Cf capacitor. schottky diodes are connected from the group reference to group inputs to carry the current outside of the devices

0 Girish Gangadharaiah over 8 years ago in reply to Vish Nadarajah

Intellectual 515 points

Hey Vish,

Can you explain it with the schematic about short circuit and flow of current. I am not able to visualize it properly.
Thanks in advance.

0 WM5295 over 8 years ago in reply to Girish Gangadharaiah

TI__Guru 54715 points

Hi Girish,

Please see the application report www.ti.com/lit/slua749 section 3

0 Girish Gangadharaiah over 8 years ago in reply to WM5295

Intellectual 515 points

Hey WM5295,

Thanks for the link, I understood the design.

0 Girish Gangadharaiah over 8 years ago in reply to Vish Nadarajah

Intellectual 515 points

Hey Vish,

Thanks for the reply, I understood the design.
But can you tell me, If we can replace Schottky diodes with Normal Diode?

0 Vish Nadarajah over 8 years ago in reply to Girish Gangadharaiah

TI__Expert 6085 points

Hi Grish,

in this network, the purpose of the Schottky is prevent the part from going below -0.3V ( the d/s abs -0.3V to 7.2V single ended) at the expense of higher leakage.

use of the conventional diode, will allow the device to go to ( - 0.7V) thus violating the device abs max spec.

the Schottky is preferred for the lower fwd turn-on ( 0.2V- 0.3V), instead of the conv diode's 0.5V-0.6V forward bias. Fast recovery of Schottky is preferred over the conventional diode

0 Girish Gangadharaiah over 8 years ago in reply to Vish Nadarajah

Intellectual 515 points

Hi Vish,

Thank you for quick reply.

I have two more doubts using diodes in different places.
1. TVS Diodes used at D38, D39 & D40, can I used Zener diodes instead of TVS Diode? Is there any reason we use TVS diodes here?
2. Fast switching diodes D22, D23 & D24, Can I use Schottky or other type of diode in this place? Is there any reason we use Fast switching diodes here?

0 Girish Gangadharaiah over 8 years ago in reply to Vish Nadarajah

Intellectual 515 points

Hi Vish,

Thank you for quick reply.

And I have 2 more doubts regarding other diodes used in the circuit

1. Why are 100 V diodes used at D22, D23 & D24, why can't we use lower voltage rated diodes since the connection done between VC5-VC5X, VC10-VC10X & VC15 and BAT. And can we use other types of diodes here like Zener or Schottky ?

2. Instead of TVS diodes at D38, D39 & D40, can we use other type of diode. Any significance of using TVS diode here?

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BQ76940EVM - Why Schottky diode are only present from 6th Cell to 15th Cell and not in first 5 Cells