L7985ATR datasheet(16/42 Pages) STMICROELECTRONICS

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Application information

L7985

16/42

DocID022446 Rev 7

Equation 10

neglecting the small ESR of the ceramic capacitors.

Considering

= 1, this function has its maximum in D = 0.5, therefore, given the maximum

peak-to-peak input voltage (VPP_MAX), the minimum input capacitor (CIN_MIN) value is:

Equation 11

Typically CIN is dimensioned to keep the maximum peak-peak voltage in the order of 1% of

VINMAX.

In Table 6 some multi-layer ceramic capacitors suitable for this device are reported.

A ceramic bypass capacitor, as close to the VCC and GND pins as possible, so that

additional parasitic ESR and ESL are minimized, is suggested in order to prevent instability

on the output voltage due to noise. The value of the bypass capacitor can go from 100 nF to

1 µF.

6.2

Inductor selection

The inductance value fixes the current ripple flowing through the output capacitor. So the

minimum inductance value, in order to have the expected current ripple, must be selected.

The rule to fix the current ripple value is to have a ripple at 20% - 40% of the output current.

In continuous current mode (CCM), the inductance value can be calculated by the following

equation:

Equation 12

where TON is the conduction time of the internal high-side switch and TOFF is the conduction

time of the external diode [in CCM, FSW = 1 / (TON + TOFF)]. The maximum current ripple, at

fixed VOUT, is obtained at maximum TOFF which is at minimum duty cycle (see Section 6.1 to

calculate minimum duty). So, by fixing

IL = 20% to 30% of the maximum output current, the

minimum inductance value can be calculated:

Table 6. Input MLC capacitors

Manufacture

Series

Cap value (

F)

Rated voltage (V)

Taiyo Yuden

UMK325BJ106MM-T

GMK325BJ106MN-T

Murata

GRM32ER71H475K

4.7

CIN

VPP FSW



---------------------------



----

–



 D D



----

–





CIN_MIN

2VPP_MAX FSW



------------------------------------------------

I

VIN VOUT

–

------------------------------ TON



VOUT VF

---------------------------- TOFF



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