AN628 datasheet(13/35 Pages) STMICROELECTRONICS

13 / 35 page

13/35

AN628 APPLICATION NOTE

This time (tSS) depends on the application parameters (output voltage, input voltage, output capacitor value,

boost inductor size, etc.) and normally the value amounts at some tens of msec.

Pin 13. VVA-OUT (Error amplifier output). Output of the E/A that determinates the control of the boosted regulated

voltage (VO). This pin has to be connected with a compensation network to the pin 14 (see fig.16).

Figure 16.

Figure 16a.

First of all, the system does not have to attempt to regulate the twice mains frequency output voltage ripple

(

∆VO) to avoid the line current distorsion. Moreover the system stability has to be ensured. The voltage open

loop gain can be splitt in two separated blocks.

The first block small signal gain, is given by the ratio between the E/A output voltage (vea) and output voltage

variation (vo) and is defined by the E/A network:

Where Gea` is the E/A gain without Rr ref. fig. 16. R2 has no effect on the error amplifier gain because the in-

verting input potential is fixed to VREF. The Gea can be seen also as the ratio between the error amplifier output

ripple and the imposed output voltage ripple (

∆VO). The E/A output signal can swing between 1.28V to 5.1V. A

value less than 2.5% of the effective E/A output swing voltage (VVAOUT = 3.82V) could be chosen to fix the Cr.

So, the Gea defined at the output voltage ripple frequency, determinates the Cr value to ensure the 100/120 Hz

(2f) attenuation.

where: Ka = 1/60 for 50Hz and 1/72 for 60Hz mains frequency.

Lower Cr value could increase harmonic distortion.The second block (Power block) is represented by the output

filter capacitor (CO) with its own reactance (XCO), the system has to be able to compensate the total external

load variation through the E/A output response (

∆Vea). The power gain transfer function (Gpw), for large varia-

tions can be written:

The total load variation (IO) to be considered is: PO(max)/VO:

Gea'

vea

----------

sR1 C

⋅⋅

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

Gea

0.095V

∆

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

≤

π 2f R1 Gea

⋅⋅

⋅

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

∆

-----------

⋅

≥

∆

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

⋅

AN628 数据表(PDF) 13 Page - STMicroelectronics

AN628 数据表(HTML) 13 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35

数据表下载