L6983_V01 datasheet(32/63 Pages) STMICROELECTRONICS

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CIN= IOUT

VPP∙FSW∙ 1−Dη ∙Dη

(36)

Considering η = 1 this function has its maximum in D = 0.5:

CINmin= IOUT

4∙VPPMAX∙FSW

(37)

Typically, CIN is dimensioned to keep the maximum peak-peak voltage across the input filter in the order of 5%

VINMAX.

In the following table, some suitable capacitor part numbers are listed.

Table 9. Capacitor part numbers

Manufacturer

Series

Size

Cap value (µF)

Rated voltage (V)

TDK

CGA5L3X5R1H106K160AB

1206

C3216X5R1H106K160AB

1206

Murata

GRT31CR61H106KE01

1206

8.5.2

Inductor selection

The inductor current ripple flowing into the output capacitor determines the output voltage ripple. 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 the following equation:

∆IL=VIN−VOUT

L ∙TON=VOUTL∙TOFF

(38)

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. So fixing ΔIL = 20% to 40% of the maximum

output current, the minimum inductance value can be calculated:

Lmin= VOUT

∆ILMAX∙1−Dmin

FSW

(39)

For those applications requiring higher inductor value for minimized current ripple, pay attention the maximum

value must prevent the sub-harmonic instability given the designed internal slope compensation. As a

consequence the inductor value must satisfy the quality factor range:

0.4≤QP≤1.33

(40)

Where QP has been defined in Section 7.1 GCO(s) control to output transfer function.

The peak current through the inductor is given by:

IL,PK=IOUT+∆IL2

(41)

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.

8.5.3

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). Therefore, 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:

∆VOUT=ESR∙∆IL,MAX+ ∆IL,MAX

8∙COUT∙FSW

(42)

For a ceramic (MLCC) capacitor, the capacitive component of the ripple dominates the resistive one. While for an

electrolytic capacitor the opposite is true. Neglecting the ESR contribution the minimum value of the output

capacitor is given by:

L6983

Design of the power components

DS13116 - Rev 2

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