TPS40050

TPS40051

TPS40053

SLUS540F − DECEMBER 2002 − REVISED JUNE 2004

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21

APPLICATION INFORMATION

TPS4005X POWER DISSIPATION

The power dissipation in the TPS4005x is largely dependent on the MOSFET driver currents and the input

voltage. The driver current is proportional to the total gate charge, Qg, of the external MOSFETs. Driver power

(neglecting external gate resistance, refer to [2] can be calculated from equation (40).

P

D + Qg

V

DR

f

SW

(Watts driver)

And the total power dissipation in the TPS4005x, assuming the same MOSFET is selected for both the high-side

and synchronous rectifier is described in equation (41).

P

T +

2

P

D

V

DR

) I

Q

V

IN

(Watts)

or

P

T + 2

Qg

f

SW ) IQ

V

IN

(Watts)

where:

D IQ is the quiescent operating current (neglecting drivers)

The maximum power capability of the device’s PowerPad package is dependent on the layout as well as air

flow. The thermal impedance from junction to air, assuming 2 oz. copper trace and thermal pad with solder and

no air flow.

q

JA + 36.515

O

C W

The maximum allowable package power dissipation is related to ambient temperature by equation (44).

P

T +

T

J * TA

q

JA

(Watts)

Substituting equation (37) into equation (35) and solving for fSW yields the maximum operating frequency for

the TPS4005x. The result is described in equation (45).

f

SW +

T

J*TA

q

JA

V

DD

* I

Q

2

Qg

(Hz)

(40)

(41)

(42)

(43)

(44)

(45)