A7985ATR datasheet(18/44 Pages) STMICROELECTRONICS

18 / 44 page

Application information

A7985A

18/44

DocID023128 Rev 5

In this case, the equation of CIN as a function of the target VPP can be written as follows:

Equation 11

neglecting the small ESR of 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 12

Typically, CIN is dimensioned to keep the maximum peak-to-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 13

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, that 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 MLCC capacitors

Manufacturer

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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