5.7

Application design guidelines

5.7.1

Input capacitor selection

The input capacitor must be rated for the maximum input operating voltage and the maximum expected RMS

input current.

Since the step-down converters' input current is a sequence of pulses from 0A to IOUT, the input capacitor must

absorb the equivalent RMS current which can be up to the load current divided by two (worst case, with duty cycle

of 50%). For this reason, the quality of these capacitors must be very high to minimize the power dissipation

generated by the internal ESR, thereby improving system reliability and efficiency.

The RMS input current (flowing through the input capacitor) is roughly estimated by:

ICIN,RMS≅IOUT⋅ D⋅ 1−D

(4)

The amount of the input voltage ripple can be roughly estimated by Eq. (5).

VIN,PP=D⋅ 1−D ⋅IOUT

CIN⋅FSW +RES,IN⋅IOUT

(5)

The suggested component is a ceramic MLCC capacitor with value 1 µF or higher, with adequate voltage rating

5.7.2

Inductor selection

mA typ., as described in Section 5.2.1 ). In LCM, to achieve a smooth transition from discontinuous to

continuous operation, i.e. from pulse skipping to constant frequency working mode, the inductor should be

L=VOUT⋅ 1−VOUTVIN

ISKIP⋅FSW

(6)

In low noise mode (LNM) the inductance value is typically selected in order to keep the current ripple in the range

20% - 40% of the maximum DC output current. However, in order to prevent the sub-harmonic instability in the

peak current mode control loop, a fixed slope compensation mechanism is implemented in L7983 by adding a

current ramp to the sensed current (see Figure 4). This approach is effective if the inductor current ripple, in the

expected input voltage range, is comparable with the above-mentioned added slope.

In conclusion, Eq. (6) is the reference design equation for inductor selection, independent of selected working

mode (LNM or LCM).

5.7.3

Output capacitor selection

In LNM working mode, the current in the output capacitor has a triangular waveform which generates a voltage

ripple across it. This ripple is due to the capacitive component (charge and discharge of the output capacitor) and

the resistive component (due to the voltage drop across its ESR). The output capacitor must be selected in order

to have a voltage ripple compliant with the application requirements.

The amount of the voltage ripple can be estimated starting from the current ripple obtained by the inductor

ΔVOUT,PP,LNM≈ΔIL⋅RES,OUT+ ΔIL

8⋅FSW⋅COUT

(7)

The ESR contribution is usually negligible in case of multi-layer ceramic capacitor (MLCC), which is the most

common choice for the L7983 typical solution. Neglecting the ESR contribution, the minimum value of the output

capacitor to guarantee the target output voltage ripple specification in LNM is estimated by:

L7983

Application design guidelines

DS13354 - Rev 1

page 23/43