L6984 datasheet(25/34 Pages) STMICROELECTRONICS

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L6984

Design of the power components

In the table below some inductor part numbers are listed.

4.3

Output capacitor selection

4.3.1

Output voltage ripple

The triangular shape current ripple (with zero average value) flowing into the output

capacitor gives the output voltage ripple, that depends on the capacitor value and the

equivalent resistive component (ESR). As a consequence the output capacitor has to be

selected in order to have a voltage ripple compliant with the application requirements.

The voltage ripple equation can be calculated as:

Equation 22

Usually the resistive component of the ripple can be neglected if the selected output

capacitor is a multilayer ceramic capacitor (MLCC).

For example with VOUT = 3.3 V, VIN = 12 V, IL = 0.12 A, fSW = 600 kHz (resulting by the

inductor value) and COUT = 4.7 F MLCC:

Equation 23

The output capacitor value has a key role to sustain the output voltage during a steep load

transient. When the load transient slew rate exceeds the system bandwidth, the output

capacitor provides the current to the load. In case the final application specifies a high slew

rate load transient, the system bandwidth must be maximized and the output capacitor has

to sustain the output voltage for time response shorter than the loop response time.

Table 7. Inductors

Manufacturer

Series

Inductor value (

H)

Saturation current (A)

Coilcraft

LPS6225

47 to 150

0.98 to 0.39

LPS5030

10 to 47

1.4 to 0.5

V

OUT

ESR

I

MAX



I

MAX

8COUT fSW



-------------------------------------

V

OUT

VOUT

------------------

VOUT

--------------

I

MAX

8COUT fSW



-------------------------------------





3.3

--------

0.12

84.7

F600kHz



-------------------------------------------------







5mV

3.3

-------------

0.15%

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