L6983 datasheet(32/63 Pages) STMICROELECTRONICS

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In case of negligible ESR (MLCC capacitor), the equation of CIN as a function of the target VPP can be written as

follows:

CIN= IOUT

VPP∙FSW∙ 1−Dη ∙Dη

(37)

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

CINmin= IOUT

4∙VPPMAX∙FSW

(38)

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

(39)

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

(40)

In order to prevent the sub-harmonic instability in the peak current control loop, a slope compensation mechanism

is internally implemented. This implies two limits on the inductor selection. In order to avoid poor slope

compensation or over slopping with a pole coming back into the BW degrading phase margin, the inductor should

be designed so that:

0.4≤QP≤1.33

(41)

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

Given the QP defined range the inductor should be selected with a value between

LSLOPE,min=VIN,MAX−VOUT

Se ∙ 1π∙QP,MAX+12

1− VOUT

VIN,MAX −1

(42)

and:

LSLOPE,MAX=VIN,min−VOUT

Se ∙ 1π∙QP,min+12

1− VOUT

VIN,min −1

(43)

In order to have the correct ripple, avoid sub-harmonic instability or over slopping, the selected inductor should

satisfy every reported conditions.

The peak current through the inductor is given by:

L6983

Design of the power components

DS13116 - Rev 1

page 32/63

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