VIPER35 datasheet(33/44 Pages) STMICROELECTRONICS

33 / 44 page

DocID026980 Rev 4

33/44

VIPer35

Operation description

In Figure 39 CFB capacitor, connected to FB pin, is used as part of the circuit to compensate

the feedback loop but it is also an element to delay OLP shutdown owing to the time needed

to charge the capacitor (see Equation 8).

After the start-up time, tSU, during which the feedback voltage is fixed at VFBlin, the output

capacitor could not be at its nominal value and the controller detects this situation as an

overload condition. In this case, OLP delay avoids the wrong device shutdown during the

start-up.

Owing to the above considerations, OLP delay time must last to bypass the initial output

voltage transient and check the overload condition only when the output voltage is in

steady-state. The output transient time depends on the value of the output capacitor and on

the load.

When CFB capacitor value is too low and cannot ensure the OLP delay, an alternative

compensation network can be used as showed in Figure 40. Two poles (fPFB, fPFB1) and

one zero (fZFB) are introduced by CFB and CFB1 capacitors and RFB1 resistor.

The capacitor CFB introduces a pole (fPFB) at higher frequency than fZB and fPFB1. This pole

compensates zero frequency due to ESR (equivalent series resistor) of the output

capacitance of the flyback converter.

By taking into account the scheme in Figure 40, these poles and zero frequency are

reported as follows:

Equation 9

Equation 10

Equation 11

RFB(DYN) is the dynamic resistance seen by FB pin and reported in Table 7.

CFB1 capacitor fixes the OLP delay and usually it is much higher than CFB. Equation 8

calculates the OLP delay time but CFB1 has to be considered. Using the alternative

compensation network, the designer can satisfy the loop stability and OLP delay time.

ZFB

π C

⋅⋅

⋅

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

PFB

FB DYN

()

FB1

π C

FB DYN

() RFB1

⋅

()

⋅⋅

⋅

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

PFB1

π C

FB1

FB DYN

()

⋅⋅

⋅

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

VIPER35 数据表(PDF) 33 Page - STMicroelectronics

VIPER35 数据表(HTML) 33 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 36 37 38 39 40 41 42 43 44

数据表下载

链接网址

VIPER35 数据表(PDF) 33 Page - STMicroelectronics

VIPER35 数据表(HTML) 33 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 36 37 38 39 40 41 42 43 44

数据表 下载

链接网址

数据表下载