PM6670AS datasheet(42/54 Pages) STMICROELECTRONICS

42 / 54 page

Application information

PM6670AS

42/54

The saturation current of the inductor should be greater than IL,PEAK not only in case of hard

saturation core inductors. Using soft-ferrite cores it is possible (but not advisable) to push

the inductor working near its saturation current.

In Table 6 some inductors are listed.

In Pulse-Skip Mode, low inductance values produce a better efficiency versus load curve,

while higher values result in higher full-load efficiency because of the smaller current ripple.

8.1.2

Input capacitor selection

In a buck topology converter the current that flows through the input capacitor is pulsed and

with zero average value. The RMS input current can be calculated as follows:

Equation 33

Neglecting the second term, the equation 10 is reduced to:

Equation 34

The losses due to the input capacitor are thus maximized when the duty-cycle is 0.5:

Equation 35

The input capacitor should be selected with a RMS rated current higher than ICINRMS(max).

Tantalum capacitors are good in terms of low ESR and small size, but they occasionally can

burn out if subjected to very high current during operation. Multi-Layers-Ceramic-Capacitors

(MLCC) have usually a higher RMS current rating with smaller size and they remain the best

choice. The drawback is their quite high cost.

Table 6. Evaluated inductors (@fsw = 400 kHz)

Manufacturer

Series

Inductance (ìH)

+40 °C rms

current (A)

-30 % saturation

current (A)

COILCRAFT

MLC1538-102

13.4

21.0

COILCRAFT

MLC1240-901

0.9

12.4

24.5

COILCRAFT

MVR1261C-112

1.1

WURTH

7443552100

COILTRONICS

HC8-1R2

1.2

16.0

25.4

LOAD

RMS

Cin

)

(

)

(

∆

⋅

−

⋅

)

(

LOAD

RMS

Cin

−

⋅

LOAD

Cin

CinRMS

Cin

loss

(max))

(

ESR

(max)

ESR

⋅

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