(

)

(

)

=

-

´

TH

TOT

A max

J max

T

T

R

P

=

+

´

J

A

TH

TOT

T

T

R

P

TOT

COND

SW

GD

Q

P

P

P

P

P

0.958 W

0.118 W

0.014 W

0.0018 W

1.092 W

=

+

+

+

=

+

+

+

=

=

´

=

´

m

=

Q

IN

Q

P

V

I

12 V

146

A

0.0018 W

GD

IN

G

SW

P

V

Q

12 V

3nC

400 kHz

0.014 W

=

´

´

=

´

´

=

f

SW

IN

SW

OUT

rise

P

V

I

t

12 V

400 kHz

5 A

4.9 ns

0.118 W

=

´

´

´

=

´

´

´

=

f

(

)

( )

2

2

OUT

COND

OUT

DS on

IN

V

5 V

P

I

R

5 A

92 m

0.958 W

V

12 V

æ

ö

=

´

´

=

´

W ´

=

ç

÷

è

ø

TPS54560

SLVSBN0 – MARCH 2013

www.ti.com

Power Dissipation Estimate

The following formulas show how to estimate the TPS54560 power dissipation under continuous conduction

mode (CCM) operation. These equations should not be used if the device is operating in discontinuous

conduction mode (DCM).

The power dissipation of the IC includes conduction loss (PCOND), switching loss (PSW), gate drive loss (PGD) and

supply current (PQ). Example calculations are shown with the 12 V typical input voltage of the design example.

(49)

spacer

(50)

spacer

(51)

spacer

(52)

Where:

IOUT is the output current (A).

RDS(on) is the on-resistance of the high-side MOSFET (Ω).

VOUT is the output voltage (V).

VIN is the input voltage (V).

fsw is the switching frequency (Hz).

trise is the SW pin voltage rise time and can be estimated by trise = VIN x 0.16 ns/V + 3 ns

QG is the total gate charge of the internal MOSFET

IQ is the operating nonswitching supply current

Therefore,

(53)

For given TA,

(54)

For given TJMAX = 150°C

(55)

Where:

Ptot is the total device power dissipation (W).

TA is the ambient temperature (°C).

TJ is the junction temperature (°C).

RTH is the thermal resistance of the package (°C/W).

TJMAX is maximum junction temperature (°C).

TAMAX is maximum ambient temperature (°C).

There will be additional power losses in the regulator circuit due to the inductor ac and dc losses, the catch diode

and PCB trace resistance impacting the overall efficiency of the regulator.

32

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