A6986 datasheet(52/67 Pages) STMICROELECTRONICS

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Application notes

A6986

52/67

DocID026211 Rev 6

7.5.3

Output capacitor selection

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 53

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

capacitor is a multi layer ceramic capacitor (MLCC).

The output capacitor is important also for loop stability: it determines the main pole and the

zero due to its ESR. (see Section 6: Closing the loop on page 35 to consider its effect in the

system stability).

For example with VOUT = 3.3 V, VIN = 12 V, IL = 0.6 A, fSW = 500 kHz (resulting by the

inductor value) and COUT = 10 F MLCC:

Equation 54

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 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.

In Table 12 some capacitor series are listed.

Table 12. Output capacitors

Manufacturer

Series

Cap value (

F)

Rated voltage (V)

ESR (m

)

MURATA

GRM32

22 to 100

6.3 to 25

< 5

GRM31

10 to 47

6.3 to 25

< 5

PANASONIC

ECJ

10 to 22

6.3

< 5

EEFCD

10 to 68

6.3

15 to 55

SANYO

TPA/B/C

100 to 470

4 to 16

40 to 80

TDK

C3225

22 to 100

6.3

< 5

V

OUT

ESR



 I

LMAX

I

LMAX

8COUT fSW



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

V

OUT

VOUT

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

VOUT

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

I

LMAX

COUT fSW



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





------



810

F 500kHz



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







15mV

3.3

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

0.45%

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