L5981TR データシートの表示（PDF） - STMicroelectronics

部品番号

コンポーネント説明

メーカー

L5981TR

1A step-down switching regulator

STMicroelectronics 

Application informations

Table 6. Inductors

Manufacturer

Coiltronics

SUMIDA

Series

DRQ73

LD2

CDR6D28MN

CDRH105RNP

L5981

Inductor Value (µH) Saturation Current (A)

10 to 22

27 to 47

10 to 22

27 to 56

1.67 to 2.47

1.64 to 2.1

1.65 to 2.5

1.9 to 2.7

5.3

Output capacitor selection

The current in the capacitor has a triangular waveform which generates a voltage ripple

across it. This ripple is due to the capacitive component and the resistive component (ESR).

So the output capacitor has to be selected in order to have a voltage ripple compliant with

the application requirements.

The amount of the voltage ripple can be calculated starting from the current ripple obtained

by the inductor selection.

Equation 9

∆VOUT

=

ESR

⋅

∆IMAX

+

-----------∆----I--M----A---X------------

8 ⋅ COUT ⋅ fSW

Usually the resistive component of the ripple is much higher than the capacitive one, if the

output capacitor adopted is not a multi layer ceramic capacitor (MLCC) with very low ESR

value.

The output capacitor is important also for loop stability: it fixes the double LC filter pole and

the zero due to its ESR. In Chapter 5.4, it will be illustrated how to consider its effect in the

system stability.

For example with VOUT=3.3V, VIN=12V, ∆IL=0.3A (resulting by the inductor value), in order to

have a ∆VOUT=0.01·VOUT, if the multi layer capacitor are adopted, 10uF are needed and the

ESR effect on the output voltage ripple can be neglected. In case of not negligible ESR

(electrolytic or tantalum capacitors), the capacitor is chosen taking into account its ESR

value. So 100uF with ESR=40mΩ is compliant with the requested output voltage ripple.

The output capacitor is also important to sustain the output voltage when a load transient

with high slew rate is required by the load. When the load transient slew rate exceeds the

system bandwidth the output capacitor provides the current to the load. So if the high slew

rate load transient is required by the application the output capacitor and system bandwidth

have to be chosen in order to sustain the load transient .

In the table below some capacitor series are listed.

Table 7. Output capacitors

Manufacturer

Series

MURATA

GRM32

GRM31

Cap value (µF)

22 to 100

10 to 47

Rated voltage (V)

6.3 to 25

6.3 to 25

ESR (mΩ)

<5

<5

16/37

Share Link: