ST1S32 datasheet(17/29 Pages) STMICROELECTRONICS

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ST1S32

Application information

Doc ID 023246 Rev 1

17/29

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

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

value. To assure the sub-harmonic stability:

Equation 20

where Vpp is the peak-to-peak value of the slope compensation ramp.

The inductor value selected, based on Equation 19, must satisfy Equation 20.

The peak current through the inductor is given by:

Equation 21

So if the inductor value decreases, the peak current (that must be lower than the current

limit of the device) increases. The higher the inductor value, the higher the average output

current that can be delivered, without reaching the current limit.

In Table 7 some inductor part numbers are listed.

5.3

Output capacitor selection

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 or discharge of the

output capacitor) and the resistive component (due to the voltage drop across its ESR). So

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 calculated starting from the current ripple obtained

by the inductor selection.

Table 7.

Inductors

Manufacturer

Series

Inductor value (µH)

Saturation current (A)

Coilcraft

XAL50xx

1.2 to 3.3

6.3 to 9

XAL60xx

2.2 to 5.6

7.4 to 11

MSS1048

1.0 to 3.8

6.5 to 11

Wurth

WE-HCI 7030

1.5 to 4.7

7 to 14

WE-PD type L

1.5 to 3.5

6.4 to 10

Coiltronics

DR73

1.0 to 2.2

5.5 to 7.9

DR74

1.5 to 3.3

5.4 to 8.35

out

•

()

⁄

LPK

ΔI

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