VIPER25HDTR datasheet(28/40 Pages) STMICROELECTRONICS

28 / 40 page

Operation description

VIPER25

28/40

Doc ID 15585 Rev 4

7.12

Summary on ZCD pin

Referring to the Figure 32 on page 26, the circuitry connected to the ZCD pin enables to

implement the following functions:

Current limit, IDlim, set point

Line feed-forward compensation

Output overvoltage protection (OVP)

Zero current detection for QR operation

Chosen RLIM, RFF and ROVP as described in previous paragraphs this function are

automatically defined.

Table 9

refers to the Figure 32 and list the external resistance combinations needed to

activate one or more functions associated to the ZCD pin.

7.13

Feedback and overload protection (OLP)

The feedback pin (FB) controls the PWM operation, enters the burst mode and manages the

delayed overload protection.

The thresholds VFBbm and VFBlin (reported on Table 8 on page 8) are respectively the low

and the high limit of the PWM operations, where the drain current is sensed trough the

integrated resistor RSENSE and applied to the comparator PWM. The PWM logic turns OFF

the power MOSFET as soon as the sensed voltage is equal to the voltage applied to the FB

pin and trough the integrated resistors network, see the Figure 2 on page 4 and Figure 20

on page 14

As shows the IC block diagram reported in Figure 2 on page 4, in parallel with the PWM

comparator there is the OCP comparator that limits the drain current as maximum to the

value IDlim, reported on Table8 on page8.

In case of higher load the voltage VFB increases, when it reaches the threshold VFBlin the

drain current is limited to IDlim and the internal current starts the charge of the capacitor CFB.

As soon as the voltage VFB reaches the threshold VFBolp, see Figure 36 on page 31, the

protection turns off the IC. After, the auto-restart mode is activated using the low value of the

current IDDch, see Table 7 on page 7.

Table 9.

ZCD pin configurations

Function / component

RLIM

ROVP

RFF

DOVP

IDlim set point

See Equation 4

Required for ZCD

Not required

Yes

OVP

22 k

See equation 7

Not required

Yes

Line feed-forward

22 k

Required for ZCD See Equation 3

Yes

IDlim set point and OVP

See Equation 4

with RFF = ∞

See equation 7

Not required

Yes

OVP and line feed-forward

22 k

See equation 7

See Equation 3

Yes

IDlim set point and line feed-forward

See Equation 4

Required for ZCD See Equation 3

Yes

IDlim reduction+ OVP +

Line feed-forward

See Equation 4

See equation 7

See Equation 3

Yes

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