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ICE2QS03 Datasheet(PDF) 10 Page - Infineon Technologies AG |
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ICE2QS03 Datasheet(HTML) 10 Page - Infineon Technologies AG |
10 / 19 page Quasi-Resonant PWM Controller ICE2QS03 Functional Description Version 2.1 10 April 21, 2011 required maximum VCS versus various input bus voltage can be calculated, which is shown in Figure 6. 0.6 0.7 0.8 0.9 1 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 Vin(V) Figure 6 Variation of the VCS limit voltage according to the IZC current According to the typical application circuit, when MOSFET is turned on, a negative voltage proportional to bus voltage will be coupled to auxiliary winding. Inside ICE2QS03, an internal circuit will clamp the voltage on ZC pin to nearly 0V. As a result, the current flowing out from ZC pin can be calculated as [5] When this current is higher than IZC_1, the amount of current exceeding this threshold is used to generate an offset to decrease the maximum limit on VCS. Since the ideal curve shown in Figure 6 is a nonlinear one, a digital block in ICE2QS03 is implemented to get a better control of maximum output power. Additional advantage to use digital circuit is the production tolerance is smaller compared to analog solutions. The typical maximum limit on VCS versus the ZC current is shown in Figure 7. 0.6 0.7 0.8 0.9 1 300 500 700 900 1100 1300 1500 1700 1900 2100 Iz c(uA) Figure 7VCS-max versus IZC 3.5 Active Burst Mode Operation At light load condition, the IC enters Active Burst Mode operation to minimize the power consumption. Details about Active Burst Mode operation are explained in the following paragraphs. 3.5.1 Entering Active Burst Mode Operation For determination of entering Active Burst Mode operation, three conditions apply: • the feedback voltage is lower than the threshold of VFBEB(1.25V). Accordingly, the peak current sense voltage across the shunt resistor is 0.17; • the up/down counter is 7; and • a certain blanking time (tBEB). Once all of these conditions are fulfilled, the Active Burst Mode flip-flop is set and the controller enters Active Burst Mode operation. This multi-condition determination for entering Active Burst Mode operation prevents mistriggering of entering Active Burst Mode operation, so that the controller enters Active Burst Mode operation only when the output power is really low during the preset blanking time. 3.5.2 During Active Burst Mode Operation After entering the Active Burst Mode the feedback voltage rises as VOUT starts to decrease due to the inactive PWM section. One comparator observes the feedback signal if the voltage level VBH (3.6V) is exceeded. In that case the internal circuit is again activated by the internal bias to start with swtiching. Turn-on of the power MOSFET is triggered by the timer. The PWM generator for Active Burst Mode operation composes of a timer with a fixed frequency of 52kHz, typically, and an analog comparator. Turn-off is resulted by comparison of the voltage signal v1 with an internal threshold, by which the voltage across the shunt resistor VcsB is 0.34V, accordingly. A turn-off can also be triggered by the maximal duty ratio controller which sets the maximal duty ratio to 50%. In operation, the output flip-flop will be reset by one of these signals which come first. If the output load is still low, the feedback signal decreases as the PWM section is operating. When feedback signal reaches the low threshold VBL(3.0V), the internal bias is reset again and the PWM section is disabled until next time regultaion siganl increases beyond the VBH threshold. If working in Active Burst Mode the feedback signal is changing like a saw tooth between 3.0V and 3.6V shown in Figure 7. 3.5.3 Leaving Active Burst Mode Operation The feedback voltage immediately increases if there is a high load jump. This is observed by one comparator. As the current limit is 34% during Active Burst Mode a certain load is needed so that feedback voltage can exceed VLB (4.5V). After leaving active busrt mode, maximum current can now be provided to stabilize VO. In addition, the up/down counter will be set to 1 ------------------------- = |
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