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# LT3957A Power Dissipation

Could you please verify how much an LT3957A with the following requirements will dissipate, please?

Vin=17-32.5V

Vout=36V

Iout=0.7A

Could you please provide each dissipation term of the formula (Pic) at pag.13 of its datasheet?

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• Hi dtluigi,

Isw is defined on page 12 as the internal switch current, which is the current flowing through the internal MOSFET M1 inside the IC.

Vpeak is supposed to be Vsw(peak). It was a typo.

Now, let me break down the formula for you.

Isw^2*D*Rds,on: ohmic power loss on the internal resistor of the MOSFET M1

Vsw(peak)^2*Isw*f*200pF/A = 1/2*Vsw(peak)^2*C is the power loss on the drain-to-source parasitic cap of M1, where C = Isw*400pF/A is the parasitic capacitance depending on Isw.

Vin*1.6mA is the input power of the IC at no load where 1.6mA is the quiescent current (IC input current at no load). It should say 1.7mA instead of 1.6mA. This is also a typo.

Vin*f*10nC is the power loss on the gate of M1, where 10nC is the total charge required to turn on M1.

I hope this helps.

Regards,

Peter Pham

• Hi dtluigi,

Isw is defined on page 12 as the internal switch current, which is the current flowing through the internal MOSFET M1 inside the IC.

Vpeak is supposed to be Vsw(peak). It was a typo.

Now, let me break down the formula for you.

Isw^2*D*Rds,on: ohmic power loss on the internal resistor of the MOSFET M1

Vsw(peak)^2*Isw*f*200pF/A = 1/2*Vsw(peak)^2*C is the power loss on the drain-to-source parasitic cap of M1, where C = Isw*400pF/A is the parasitic capacitance depending on Isw.

Vin*1.6mA is the input power of the IC at no load where 1.6mA is the quiescent current (IC input current at no load). It should say 1.7mA instead of 1.6mA. This is also a typo.

Vin*f*10nC is the power loss on the gate of M1, where 10nC is the total charge required to turn on M1.

I hope this helps.

Regards,

Peter Pham

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