VIPER27HD datasheet(20/31 Pages) STMICROELECTRONICS

20 / 31 page

Operation descriptions

VIPER27

20/31

Doc ID 15133 Rev 5

7.9

About CONT pin

Referring to the

Figure 27, through the CONT pin, the below features can be implemented:

Current limit set point

Over voltage protection on the converter output voltage

The

Table 9 on page 20 referring to the Figure 27, lists the external components needed to

activate one or plus of the CONT pin functions.

Figure 27.

CONT pin configuration

7.10

Feed-back and overload protection (OLP)

The VIPER27 is a current mode converter: the feedback pin controls the PWM operation,

controls the burst mode and actives the overload protection.

Figure 28 on page 22 and

Figure 29 show the internal current mode structure.

With the feedback pin voltage between VFBbm and VFBlin, (see Table 8 on page 7) the drain

current is sensed and converted in voltage that is applied to the non inverting pin of the

PWM comparator. See

Figure 2 on page 3.

This voltage is compared with the one on the feedback pin through a voltage divider on

cycle by cycle basis. When these two voltages are equal, the PWM logic orders the switch

off of the power MOSFET. The drain current is always limited to IDlim value.

In case of overload the feedback pin increases in reaction to this event and when it goes

higher than VFBlin, the PWM comparator is disabled and the drain current is limited to IDlim by

the OCP comparator, see

Figure 2 on page 3.

Table 9.

CONT pin configurations

Function / component

RLIM

(1)

RLIM has to be fixed before of ROVP

ROVP

DAUX

IDlim reduction

See

Figure 16

OVP

≥ 80 kΩ

See

Equation 4

Yes

IDlim reduction + OVP

See

Figure 16

See

Equation 4

Yes

OVP

ROV P

SOFT

START

CONT

Daux

RLIM

From RSENSE

Auxiliary

winding

to GATE driver

OVP

LOGIC

OCP

BLOCK

OCP

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