AN4501 datasheet(11/48 Pages) STMICROELECTRONICS

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DocID026394 Rev 1

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AN4501

Guideline for the selection of external components

Two 4.7 µF MLCC with a voltage capability of 50 V and with an ESR of 4 m

Ω can be placed

in parallel. The resulting ESR is 2 m

Ω (ESR

COUT). The resulting maximum output current

ripple due to the bulk capacitance is:

The additional current ripple caused by the ESR is:

which is indeed negligible compared to IOUT,PP,MAX.

The total output current ripple is 62 mA, fulfilling the target.

Step 5: Selection of the input capacitor

We assume that MLCC with a very low ESR is used. The contribution of the ESR to the

input voltage ripple can be neglected as well, compared to the contribution of the charge

and discharge of the input capacitor itself.

The maximum inductor peak-to-peak current IL,PP,MAX is:

The minimum input capacitor is calculated by:

Using two 4.7 µF, 50 V input capacitor with an ESR of 4m

Ω (ESR

CIN), the maximum input

voltage ripple caused by the bulk capacitance is:

OUT,MIN

LED

DLED

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

OUT

MAX

OUT,PP,MAX,TARGET

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

OUT,MIN

80.4

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

1.0

0.791

0.075

450 10

⋅

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

7.34µF

OUT,PP,MAX

LED

DLED

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

OUT

MAX

OU T

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

80.4

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

1.0

0.791

9.4 10

–

⋅

450 10

⋅

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

58.5mA

ESR

COUT

L,PEAK,MAX

LED

DLED

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

210

–

5.59

⋅

80.4

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

4.7mA

L,PP,MAX

OUT

–

()

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

0.5

6.8 10

–

450 10

0.9

⋅

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

2.36A

IN,MIN

L,PP,MAX

IN,PP,MAX,TARGET

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

2.36

450 10

0.100

⋅

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

6.6µF

L,PP,MAX

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

2.36

450 10

9.4 10

–

⋅

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

70mV

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