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BQ24040: some questions about the power dissipation and ISET

Alice_chen

Prodigy 120 points

Part Number: BQ24040
Other Parts Discussed in Thread: TPS7A02

Hi team:

1.I found this IC is a linear charger device,but I can't find it's power dissipation ,can i calculate in this way?

PD=(125-25)/63.5=1.57W

2.If i pull low the ISET2,and set the current to 1000mA by the pin ISET.The current will always keep 1A in fast-charge phase?

3.At the beginning of the fast-charge,the PD exceed 1.57W((5-2.5)*1=2.5W),what will IC do next ? Enter thermal requlation?

Looking fordward to your reply!

Alice

over 4 years ago

0 Nick Brylski over 4 years ago

TI__Mastermind 19195 points

Hi Alice,

Power dissipation normally calculated P = (Vin - Vbat) * Current. If IC is at 125C can estimate using Junction-to-board characterization parameter, P = (125C-25C) / 34.3 C/W = 2.91 W
Yes, as long as the device is not getting too hot. Current begins to be reduced when around 125C.
T = 25C + 2.5W * 34.3 C/W = 111C which is under the thermal regulation. Recommend following the layout guidelines in d/s section 11 to ensure good thermals.

Best,

0 Alice_chen over 4 years ago in reply to Nick Brylski

Prodigy 120 points

Hi nick:

Thanks for your answer..

I find you use ψJB(34.3) to calculate the PD but i used θJA(63.5),which lead to the different results.

By the way,TPS7A02(also ldo) use θJA to calculate the PD as following picture,so which one is correct?

if θJA is correct,would you mind answering my questions(the first time) again?

Thanks!

Alice

0 Nick Brylski over 4 years ago in reply to Alice_chen

TI__Mastermind 19195 points

Alice,

ψJB is the correct parameter to use.

Best,

0 Nick Brylski over 4 years ago in reply to Nick Brylski

TI__Mastermind 19195 points

Please refer to 8.1.5.1 of the LDO datasheet you linked. It also uses ψJB.

0 Alice_chen over 4 years ago in reply to Nick Brylski

Prodigy 120 points

Hi nick:

What's the difference between ψJB and θJA ,why should we uses ψJB.

Isn't 25℃ the ambient temperature? (θJA)

Thanks!

alice

0 Alice_chen over 4 years ago in reply to Alice_chen

Prodigy 120 points

Hi nick:

could you please answer the above questions?

What's the difference between ψJB and θJA ,why should we uses ψJB.

Isn't 25℃ the ambient temperature? (θJA)

By the way,which equation should we use to calcluate Tj in practical application?

Thanks!

0 Nick Brylski over 4 years ago in reply to Alice_chen

TI__Mastermind 19195 points

Hi Alice,

In a practical applications you should use ψJB, please refer to my first comment for as an example here.

What's the difference between ψJB and θJA ,why should we uses ψJB. Please refer to section 8.1.5.1 of the LDO d/s you linked.

Best,