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Peak Current Rating of Inductor

I

CHARGE +

V

OUT

C

OUT

T

SS

(9)

L

L(peak) + IOUT(max) )

1

2

I

RIPPLE ) ICHARGE

(10)

Input Capacitor Selection (C1 & C2)

C

IN(min) +

I

LOAD

V

OUT

V

RIPPLE(cap)

V

IN

F

SW

(11)

ESR

MAX +

V

RIPPLE(ESR)

I

LOAD )

1

2

I

RIPPLE

(12)

I

RMS(cin) + IIN(rms) * IIN(avg) +

I

OUT

2

) 1

12

I

RIPPLE

2

V

OUT

V

IN

*

V

OUT

I

OUT

V

IN

(13)

TPS40040, TPS40041

SLUS700D – MARCH 2006 – REVISED DECEMBER 2007

With output capacitance known, it is now possible to calculate the charging current during start-up and determine

the minimum saturation current rating for the inductor. The start-up charging current is approximated by:

Using the TPS40041’s fixed 4.5-ms soft-start time, COUT = 200 µF and VOUT = 1.8 V, ICHARGE is found to be 80

mA. The peak current rating of the inductor is now found by:

The inductor requirements are summarized in the table below.

Inductor Requirements

PARAMETER

SYMBOL

VALUE

UNITS

Inductance

L

1.0

µH

RMS current (thermal rating)

IL(rms)

6.03

A

Peak current (saturation rating)

IL(peak)

7.08

A PG0083.102, 1.0

µH is selected for its small size, low DCR and high current handling capability.

The input voltage ripple is divided between capacitance and ESR. For this design, VRIPPLE(CAP) = 50 mV and

VRIPPLE(ESR) = 25 mV. The minimum capacitance and maximum ESR are estimated by:

For this design, CIN > 80 µF and ESR < 3.5 mΩ. The RMS current in the input capacitors is estimated by:

With VIN = VIN(max), the input capacitors must support a ripple current of 1.56 ARMS. Two 1206, 100-µF, X5R

ceramic capacitors with about 5-m

Ω ESR and a 2-A RMS current rating are selected. It is important to check the

dc bias voltage derating curves to ensure the capacitors provide sufficient capacitance at the working voltage.

Copyright © 2006–2007, Texas Instruments Incorporated

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Product Folder Link(s): TPS40040 TPS40041