A6986H datasheet(51/71 Pages) STMICROELECTRONICS

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Table 15. Input capacitors

Manufacturer

Series

Size

Cap value (µF)

Rated voltage (V)

TDK

C3225X7S1H106M

1210

C3216X5R1H106M

1206

Taiyo Yuden

UMK325BJ106MM-T

1210

7.6.2

Inductor selection

The inductor current ripple flowing into the output capacitor determines the output voltage ripple (please refer to

Section 7.6.3 Output capacitor selection). Usually the inductor value is selected in order to keep the current

ripple lower than 20% - 40% of the output current over the input voltage range. The inductance value can be

calculated by equation below:

ΔIL=VIN−VOUT

L ⋅TON=VOUTL⋅TOFF

(49)

Where TON and TOFF are the on and off time of the internal power switch. The maximum current ripple, at fixed

VOUT, is obtained at maximum TOFF that is at minimum duty cycle (see Section 7.6.1 Input capacitor selection to

calculate minimum duty). So fixing ΔIL = 20% to 40% of the maximum output current, the minimum inductance

value can be calculated:

LMIN= VOUT

ΔILMAX⋅1−DMIN

fSW

(50)

where fSW is the switching frequency 1/(TON + TOFF).

For example for VOUT = 3.3 V, VIN = 12 V, IOUT = 2 A and FSW = 500 kHz the minimum inductance value to have

ΔIL = 30% of IOUT is about 8.2 µH.

The peak current through the inductor is given by:

IL,PK=IOUT+ΔIL2

(51)

So if the inductor value decreases, the peak current (that has to be lower than the current limit of the device)

increases. The higher is the inductor value, the higher is the average output current that can be delivered, without

reaching the current limit.

In the table below, some inductor part numbers are listed.

Table 16. Inductors

Manufacturer

Series

Inductor value (µH)

Saturation current (A)

Coilcraft

XAL50xx

2.2 to 22

6.5 to 2.7

XAL60xx

12.5 to 4

7.6.3

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:

ΔVOUT=ESR⋅ΔILMAX+ ΔILMAX

8⋅COUT⋅fSW

(52)

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 to consider its effect in the system stability).

A6986H

Design of the power components

DS12861 - Rev 3

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