(

)

(OFF,min)

OUT

SCHO TTKY

L

O UT

IN

OUT

DS(on)

L

OUT

ON

t

V

+ V

+ R

I

V

V

IOUT

r

R

I

t

,MIN

´

´

-

-

´

-

´

³

(

)

IN

OUT

OUT

DS(on)

L

OUT

ON

MIN

V

V

I

r

R

I

t

,MIN

L

=

I

-

-

´

-

´

´

D

(

)

O UT

S CHOTTKY

L

OUT

OFF

MIN

V

+ V

+ R

I

t

,MIN

L

=

I

´

´

D

OUT

PMOS(RMS)

OUT

OUT

IN

V

I

I

D = I

V

»

(

)

[

]

(

)

(

)

2

(cond)

OUT

DS(on)

J

OUT

DS(on)

P

= I

r

1 + TC

T

25 C

I

D

r

´

´

´

-

°

»

´

D

TPS75003-EP

SGLS311A

– DECEMBER 2006 – REVISED MARCH 2011

www.ti.com

Inductor Value Selection (Buck Controllers)

The inductor is chosen based on inductance value and maximum current rating. Larger inductors reduce current

ripple (and therefore, output voltage ripple) but are physically larger and more expensive. Inductors with lower

DC resistance typically improve efficiency, but also have higher cost and larger physical size. The buck

converters work well with inductor values between 4.7

μH and 47 μH in most applications. When selecting an

determine the minimum inductor size, first determine if the device will operate in minimum on-time or minimum

off-time mode. The device will operate in minimum on-time mode if Equation 5 is satisfied.

(5)

Minimum inductor size needed when operating in minimum on-time mode is given by Equation 6.

(6)

Minimum inductor size needed when operating in minimum off-time mode is given by Equation 7.

(7)

External PMOS Transistor Selection (Buck Controllers)

The PMOS gate will see voltages from 0 V to the maximum input voltage, so gate-to-source breakdown should

be a few volts higher than the maximum input supply. The drain-to-source of the device will also see this full

voltage swing, and should therefore be a few volts higher than the maximum input supply. The RMS current in

the PMOS can be estimated by using Equation 8.

(8)

The power dissipated in the PMOS is comprised of both conduction and switching losses. Switching losses are

are calculated by Equation 9.

(9)

16

Copyright

© 2006–2011, Texas Instruments Incorporated

Product Folder Link(s): TPS75003-EP