VIPER35 datasheet(32/44 Pages) STMICROELECTRONICS

32 / 44 page

Operation description

VIPer35

32/44

DocID026980 Rev 4

8.13

Feedback and overload protection (OLP)

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

delayed overload protection.

VFBbm and VFBlin thresholds (Table 7) are respectively low and high limit of PWM

operations, where the drain current is sensed by 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 FB pin and through the integrated

resistor network (see Figure 2 and Figure 23).

IC block diagram (Figure 2) shows in parallel with the PWM comparator how OCP

comparator limits the drain current to IDlim value, as per Table 7.

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

current is limited to IDlim by OCP comparator and the internal current starts the charge of

CFB capacitor. As soon as the voltage VFB reaches the threshold VFBolp, see Figure 41, the

protection turns off the IC. The auto-restart mode is active using the low value of the current

IDDch, see Table 7.

The time, from the high load detection, VFB = VFBlin, to the overload turn-off, VFB = VFBolp,

depends on the value of CFB capacitor and on the internal charge current, IFB. OLP delay

time can be calculated as follows:

Equation 8

The current, IFB, is 3 A as minimum value. Components, connected to FB pin, belong to the

compensation loop, so they have to be selected taking into account the proper delay and

loop stability. Figure 39 and Figure 40 show two different feedback networks.

Table 10. ZCD pin configurations

IDlim

OVP

RLim

ROVP

RFF

DOVP

■□

□

Equation 4

Equation 7 with

VOUTOVP ˃ 2 VOUT

Yes

□■

□

22 k

Ω

Equation 7

Yes

□□

■

22 k

Ω

Equation 7 with

VOUTOVP˃2 VOUT

Equation 3

Yes

■■

□

Equation 4

with RFF= ∞

Equation 7

Yes

□■

■

22 k

Ω

Equation 7

Equation 3

Yes

■□

■

Equation 4

Equation 7 with

VOUTOVP ˃2 VOUT

Equation 3

Yes

■

■■

Equation 4

Equation 7

Equation 3

Yes

OLP_delay

FBolp

FBlin

–

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