AN3392 datasheet(35/57 Pages) STMICROELECTRONICS

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AN3392

External component selection

Doc ID 018749 Rev 1

35/57

Note:

In order to guarantee the proper functionality of VOUT_SNS current flowing in the series,

connection of R3 and R4 should be between 20 µA and 200 µA.

10.10

Output voltage sensing capacitor

C10 is placed in parallel with R4 and is as close as possible to VOUT_SNS.

Its role is to maximize the stability of the voltage sensed by the VOUT_SNS pin.

If R3 and R4 are chosen properly and partition VOUT to 1.25 V, the voltage rating of this

capacitor can be 3.3 V or higher.

The capacitance value depends on the time constant (

out) composed with R4 (τout=

C8*R3//R4) and by the system switching frequency (FSSW = 4*FSW).

Assuming R4<<R3 (so, R3//R4

≅ R4):

Equation 24

so,

Equation 25

Note:

Even if the SPV1020 controls each phase at FSW (by default 100 kHz), the entire system

switching frequency (System-Fswitch) is four times the single-phase switching frequency (by

default 400 kHz).

10.11

Internal oscillator frequency

The SPV1020 controls the boost application by a PWM signal operating at the default

switching frequency of 100 kHz.

Default switching frequency is guaranteed by connecting the OSC_IN pin to 5 V (VREG).

The user can change the default value by placing a proper resistor (R6), as shown in

Figure 24, to ground.

The internal oscillator works with an integrated resistor of 120 k

Ω. Frequency is proportional

to the current provided to the oscillator block.

To change Fswitch to the desired switching frequency, R7 must be selected according to the

following formula:

Equation 26

OUT

ssw

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

⋅

≈

C10

SSW

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

--------

⋅⋅

≈

R7 k

Ω

[]

100

()

120

()

⋅

switch kHz

[]

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