AN3392 datasheet(31/57 Pages) STMICROELECTRONICS

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AN3392

External component selection

Doc ID 018749 Rev 1

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According to Figure 6, during the charge phase (switch ON), peak current on each inductor

depends on the applied voltage (Vin), on the inductance (Lx) and on the time (TON).

Equation 11

Taking into account the overcurrent threshold (4.5 A):

Equation 12

Finally, inductance should be chosen according to the following formula:

Equation 13

Considering the possible unbalance of the currents and the inductance drop due to self

heating effect, a more conservative choice would be to replace 4.5 A with 3.15 A (70%).

Triggering the overcurrent threshold will cause limitation of the duty cycle and consequently

limitation of the input and output powers.

Usually, inductances ranging between 22 µH to 100 µH satisfy most application

requirements.

Critical parameters for the inductor choice are inductance (analyzed above), Irms current,

saturation current, and size.

The current flowing through an inductor causes its internal temperature increase (self-

heating effect). Irms typically indicates the current value causing a temeprature increase of

20, 30, or 40 °C. The higher the temeprature, the higher the inductance drop with respect to

its nominal value.

For the same physical size, smaller inductance values provide for faster response to load

transients and higher efficiency. Inductor size also affects the maximum current deliverable

to the load. The saturation current of the inductor should be higher than the peak current

limit of the input source. The suggested saturation current should be > 4.5 A.

Inductors with low series resistance are suggested to guarantee high efficiency.

10.3

Bootstrap capacitors

C1, C2, C3, and C4 are four capacitors used to guarantee internal functionality of the

SPV1020. Their role is to maintain the required voltage level on pins CB1, CB2, CB3, and

CB4 even during the charging phase of the inductors.

Capacitance value is the same for all four capacitors and is not application dependant. The

suggested value is in the range of 22 nF to 100 nF. Each capacitor switches synchronously

with the related inductor (at 100 kHz). The maximum voltage is fixed by the internal voltage

regulator (~5 V).

Lx pk

() I

Lx rms

()

---

Vmp

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

⋅⋅

≅

)

(

ILx

---

4.5

Lx rms

()

–

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

---

⋅

4.5

--------

–

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

⋅

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