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MC44608 Datasheet(PDF) 7 Page - ON Semiconductor |
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MC44608 Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 16 page MC44608 http://onsemi.com 7 Figure 5. Vcont 2.4 V Iprim DMG Clock OSC 4 V The OSC and Clock signals are provided according to the Figure 5. The Clock signals correspond to the CT capacitor discharge. The bottom curve represents the current flowing in the sense resistor Rcs. It starts from zero and stops when the sawtooth value is equal to the control voltage Vcont. In this way the SMPS is regulated with a voltage mode control. Overvoltage Protection The MC44608 offers two OVP functions: – a fixed function that detects when VCC is higher than 15.4V – a programmable function that uses the demag pin. The current flowing into the demag pin is mirrored and compared to the reference current Iovp (120 µA). Thus this OVP is quicker as it is not impacted by the VCC inertia and is called QOVP. In both cases, once an OVP condition is detected, the output is latched off until a new circuit START–UP. Start–up Management The Vi pin 8 is directly connected to the HV DC rail Vin. This high voltage current source is internally connected to the VCCpinandthusisusedtochargetheVCCcapacitor.The VCC capacitor charge period corresponds to the Start–up phase. When the VCC voltage reaches 13V, the high voltage 9mA current source is disabled and the device starts working. The device enters into the switching phase. It is to be noticed that the maximum rating of the Vi pin 8 is 700V. ESD protection circuitry is not currently added to this pin due to size limitations and technology constraints. Protection is limited by the drain–substrate junction in avalanche breakdown. To help increase the application safety against high voltage spike on that pin it is possible to insert a small wattage 1k W series resistor between the Vin rail and pin 8. The Figure 6 shows the VCC voltage evolution in case of no external current source providing current into the VCC pin during the switching phase. This case can be encountered in SMPS when the self supply through an auxiliary winding is not present (strong overload on the SMPS output for example). The Figure 16 also depicts this working configuration. Figure 6. Hiccup Mode Start–up Latched off Phase Switching Phase Phase V CC 6.5 V 10 V 13 V In case of the hiccup mode, the duty cycle of the switching phase is in the range of 10%. Mode Transition The LW latch Figure 7 is the memory of the working status at the end of every switching sequence. Two different cases must be considered for the logic at the termination of the SWITCHING PHASE: 1. No Over Current was observed 2. An Over Current was observed These 2 cases are corresponding to the signal labelled NOC in case of “No Over Current” and “OC” in case of Over Current. So the effective working status at the end of the ON time memorized in LW corresponds to Q=1 for no over current and Q=0 for over current. This sequence is repeated during the Switching phase. Several events can occur: 1. SMPS switch OFF 2. SMPS output overload 3. Transition from Normal to Pulsed Mode 4. Transition from Pulsed Mode to Normal Mode Figure 7. Transition Logic & I SQ R LW + – & CS Q & R2 Q S Mode LEB out 1 V VPWM OUT Stand–by R1 Start–up Phase Switching Phase Start–up Phase NOC OC > 24 A m Latched Off Phase & demag S1 Switch • 1. SMPS SWITCH OFF When the mains is switched OFF, so long as the bulk electrolithic bulk capacitor provides energy to the SMPS, the controller remains in the switching phase. Then the peak current reaches its maximum peak value, the switching frequency decreases and all the secondary voltages are reduced. The VCC voltage is also reduced. When VCC is equal to 10V, the SMPS stops working. |
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