AN4026 datasheet(22/40 Pages) STMICROELECTRONICS

22 / 40 page

Functional check

AN4026

22/40

Doc ID 022603 Rev 1

lasting less than the set time do not cause any other action, therefore providing the system

with immunity to short duration phenomena. If, instead, overload condition keeps going, a

protection procedure is activated that shuts down the L6699 and, in the case of continuous

overload/short-circuit, results in continuous intermittent operation with a user defined duty

cycle. This function is realized with the pin DELAY (#2), by means of a capacitor C45 and

the parallel resistor R24 connected to ground. As the voltage on the ISEN pin exceeds 0.8 V,

the first OCP comparator, in addition to discharging CSS, turns on an internal 150 µA

current generator that, via the DELAY pin, charges C45. As the voltage on C45 is 3.5 V, the

L6699 stops switching and the PFC_STOP pin is pulled low. Also the internal generator is

turned off so that C45 is now slowly discharged by R24. The IC restarts when the voltage on

C45 is less than 0.3 V. Additionally, if the voltage on the ISEN pin reaches 1.5 V for any

reason (e.g. transformer saturation), the second comparator is triggered, the L6699 shuts

down and the operation is resumed once the voltage on C45 drops below 0.3 V.

In Figure 23 a dead short-circuit event has been captured. In this case the overcurrent

protection is triggered by the second comparator referenced at 1.5 V, it immediately stops

switching by the L6699 and discharging of the soft-start capacitor; at the same time the

capacitor connected to the DELAY pin (#2) begins charging up to 3.5 V (typ.). Once the

voltage on the DELAY pin reaches 3.5 V, the L6699 stops charging the delay capacitor

(C45), the L6699 operation is resumed once the DELAY pin (#2) voltage decays to 0.3 V

(typ.) by the parallel resistor (R24), via a soft-start cycle. If the short-circuit condition is

removed, the converter again starts operation, otherwise, if the short is still there, it results

in an intermittent operation (hiccup mode) with a narrow operating duty cycle of the

converter, in order to prevent the overheating of power components, as can be noted in

Figure 25.

In Figure 24 details of peak current at short-circuit occurring is captured. Note the ZVS

correct operation by the half bridge MOSFETs.

Figure 23.

Short-circuit at full load

Figure 24.

Short-circuit at full load - detail

CH1: HB voltage

CH3: CSS

CH2: LVG

CH4: DELAY

CH1: HB voltage

CH3: CSS

CH2: LVG

CH4: DELAY

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