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# About junction temperature of SN105125

Other Parts Discussed in Thread: SN105125

As soon as I look datesheet of SN105125, it doesn't write the junction temperature of recommended operating conditions. Please tell me about  junction temperature of recommended operating conditions of SN105125?

• Takahiko,

The junction temperature depends a number of different variables, such as thermal characteristics of the package and the PCB area surrounding the package, along with the power dissipation across the LDO. As such, we specify the ambient temperature around the device and the junction temperature recommended operational range.

There are quick calculations to give you really rough idea of the junction temperature, and I will walk you through these calculation:

Let’s assume we have an LDO with a Vin of 3V and a Vout of 1V, delivering a stead-state load current of 1A. We will assume Iq is negligible.

As such, the power dissipated across the LDO is given by:

P_d=(V_in-V_out )*I_out=V_dropout*I_out=(3V-1V)*1A=2W

Now this power is dissipated across the LDO in the form of heat, so with more assumptions we can say (equation from app note below): temperature

P_d=(T_J-T_A)/θ_JA

where T_j is the junction temperature,

T_A is the ambient temperature,

and θ_JA  is a measure of the heat transferred from the junction to the ambient  (°C/W)

With a quick rearrangement,

T_j=P_d*θ_JA+T_A

And if we now assume that:

θ_JA=20  (°C)/W  and T_A=25 °C

And we can quickly estimate:

T_j=2W*20 (°C)/W+25°C=65 °C

The application note below explains the theory behind the calculation above and also demonstrates how a number of variables go into finding the junction temperature:

http://www.ti.com/lit/an/slva118a/slva118a.pdf

Regards,

David Hopkins

L&S Applications Engineer

• Hi

I'sorry, I enough can't explain. Is 150℃  junction temperature to recommend?

Please refer to the figure listed below.

Best regards.

• Takahiko,

The recommended junction temperature maximum is 105.

Regards,

David

• Hi

I understand.

Thank you.