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VIPER25HD Datasheet(PDF) 28 Page - STMicroelectronics

Part No. VIPER25HD
Description  Off-line high voltage converters
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Maker  STMICROELECTRONICS [STMicroelectronics]
Homepage  http://www.st.com
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VIPER25HD Datasheet(HTML) 28 Page - STMicroelectronics

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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:
1.
Current limit, IDlim, set point
2.
Line feed-forward compensation
3.
Output overvoltage protection (OVP)
4.
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
Ω
See equation 7
Not required
Yes
Line feed-forward
22 k
Ω
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
Ω
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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