AST1S31HF datasheet(20/33 Pages) STMICROELECTRONICS

20 / 33 page

Application information

AST1S31HF

20/33

DocID026968 Rev 2

The placement of the input capacitor is very important to avoid noise injection and voltage

spikes on the input voltage pin. So the CIN must be placed as close as possible to the

VIN_SW pin. In Table 6 some multilayer ceramic capacitors suitable for this device are

given.

A ceramic bypass capacitor, as close as possible to the VINA pin 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 330 nF to 1 µF.

7.2

Inductor selection

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

minimum inductance value 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 the continuous current mode (CCM), the inductance value can be calculated by Equation

28:

Equation 28

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

the low-side switch [in CCM, FSW = 1 / (TON + TOFF)]. The maximum current ripple, given

the VOUT, is obtained at maximum TOFF, that is, at a minimum duty cycle (see previous

section to calculate minimum duty). So by fixing

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

current, the minimum inductance value can be calculated:

Equation 29

where FSWMIN is the minimum switching frequency, according to Table 4 on page 7. The

slope compensation, to prevent the sub-harmonic instability in the peak current control loop,

is internally managed and so fixed. This implies a further lower limit for the inductor value.

To assure sub-harmonic stability:

Equation 30

where VPP is the peak-to-peak value of the slope compensation ramp. The inductor value

selected based on Equation 29 must satisfy Equation 30.

Table 6. Input MLCC capacitors

Manufacturer

Series

Cap value (

µF)

Rated voltage (V)

Murata

GCM

6.3

TDK

CGA6

6.3

TAIYO YUDEN

LMK325

I

VIN VOUT

–

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



VOUT

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



LMIN

VOUT

I

MAX

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

1DMIN

–

FSWMIN

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



LVout 2Vpp fsw









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