S12ZVM current capability w/ complementary darlington

[keisukeninomiya]

Hello

S12ZVM has a capabillity to increase current from 70mA to 170mA with ballast transistor.

Now Customer is asking to increast this current more than 170mA such 300mA.

I think if complementary darlington circuit can achieve this request.

The circuit is one reference as the folloiwng URL.

Let me know if there is possibility or not.

Regards,

Nino

https://www.google.co.jp/search?q=darlington+transistor&biw=1195&bih=602&tbm=isch&imgil=zOW0z_J42rtR...

[iggi]

The complementary transistor can be a solution, but per my opinion it is unnecessary.

For example, the max. collector current of BCP53 transistor is 1.5Amps!

http://www.onsemi.com/pub_link/Collateral/BCP53T1-D.PDF

But, if the ballast transistor can supply more than 1A of current, it does not mean we can connect external loads to the MCU pins which will consume such current.

In this case, the user must ensure that the maximum junction temperature is not exceeded. It depends on ambient temperature, thermal package characteristics and MCU power dissipation.

MCU power has to be calculated for the particular application.This is explained in S12ZVM RM, Appendix A.1.7.

Regards,

iggi

[keisukeninomiya]

Hi iggi

Thank you for your feedback.

I have considered your idea with BCP53 PNP transistor.

If customer want to 300mA, regarding Fig2 in BCP53 datasheet, hfe is about 50. Then Ib=Ic/hfe=6mA

Injection current in CCS RM shows 2.5mA for each pins. Then this might be out of spec.

Am I correct ?

Regards,

Nino

[Wlodek_D_ · Accepted solution]

Hello,

Yes, if there is 300mA required, then your Customer can use PNP Darlington transistor e.g. MJD117 or MMBTA63LT1G from ON Semi (see http://www.onsemi.com/pub_link/Collateral/MJD112-D.PDF or http://www.onsemi.com/pub_link/Collateral/MMBTA63LT1-D.PDF respectively.

It will be safer for MCU to drive Darlington than single transistor with quite low hfe.

Best Regards,

Wlodek_D.

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[keisukeninomiya]

Hi Wlodek

Thanks.

Understood.

Regards,

Nino

[jrrlopez]

Hi Nino,

An additional very important item is the power dissipation considerations for the pass transistor and make sure you don't exceed the maximum power and operating temperature range. For example, if you consider a maximum voltage of 18V and considering the ballast transistor regulates down to 5V with a current demand of 300mA, you are reaching a power dissipation of 3.9W!! (18V - 5V)*0.3 which could be way beyond the power dissipation capabilities of the transistor.

Regards,

Juan