RMSC out

Vout

1

1

Vin

I

= Vout

L ×

2

3

f

-

´

´

´

Vout

1

1

Vin

Vout = Vout

+ ESR

L ×

8

Cout

f

f

-

æ

ö

´

´ ç

÷

´

´

è

ø

TPS65708

www.ti.com

SLVSAE1A

– OCTOBER 2010 – REVISED FEBRUARY 2011

Refer to Table 1 and the typical applications for possible inductors.

Table 1. Tested Inductors

Inductor type

Inductor Value

Supplier

BRC1608

1.5

µH

Taiyo Yuden

MLP2012

2.2

µH

TDK

MIPSA2520

2.2

µH

FDK

GLCR1608T1R5M-HC

1.5

µH

TDK

LQM21P

2.2

µH

Murata

Output Capacitor Selection

The advanced Fast Response voltage mode control scheme of the step-down converter allows the use of small

ceramic capacitors with a typical value of 10

µF, without having large output voltage under and overshoots during

heavy load transients. Ceramic capacitors having low ESR values result in lowest output voltage ripple and are

therefore recommended. For an inductor value of 1.5

µH or 2.2µH, an output capacitor with 10µF can be used.

See the recommended components.

If ceramic output capacitors are used, the capacitor RMS ripple current rating will always meet the application

requirements. Just for completeness the RMS ripple current is calculated as:

(3)

At nominal load currents, the inductive converters operate in PWM mode and the overall output voltage ripple is

the sum of the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and

discharging the output capacitor:

(4)

Where the highest output voltage ripple occurs at the highest input voltage Vin.

At light load currents, the converter operates in Power Save Mode and the output voltage ripple is dependent on

the output capacitor value. The output voltage ripple is set by the internal comparator delay and the external

capacitor. The typical output voltage ripple is less than 1% of the nominal output voltage.

Input Capacitor Selection

Because of the nature of the buck converter having a pulsating input current, a low ESR input capacitor is

required for best input voltage filtering and minimizing the interference with other circuits caused by high input

voltage spikes. The converters need a ceramic input capacitor of 10

μF. The input capacitor can be increased

without any limit for better input voltage filtering.

Table 2. Tested Capacitors

Type

value

Voltage rating

Size

Supplier

Material

GRM155R60G475ME47D

4.7

µF

4V

0402

Murata

Ceramic X5R

GRM155R60J225ME15D

2.2

µF

6.3V

0402

Murata

Ceramic X5R

GRM185R60J225

2.2

µF

6.3V

0603

Murata

Ceramic X5R

GRM188R60J475K

4.7

µF

6.3V

0603

Murata

Ceramic X5R

GRM188R60J106ME47D

10

µF

6.3V

0603

Murata

Ceramic X5R

© 2010–2011, Texas Instruments Incorporated

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