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44608P40 Datasheet(PDF) 6 Page - ON Semiconductor |
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44608P40 Datasheet(HTML) 6 Page - ON Semiconductor |
6 / 16 page MC44608 http://onsemi.com 6 In stand–by mode, this current can be lowered as due to the activation of a 200 µA current source: Ipk max *stby + 1 * (Rcs(kW) 0, 2) Rsense(W) (A) Figure 2. Current Sense + – Switching Phase 1 V STAND–BY 10 & 2 Isense L.E.B. Overcurrent OC 200 A m START–UP Comparator Rcs Rsense The current sense input consists of a filter (6k W, 4pF) and of a leading edge blanking. Thanks to that, this pin is not sensitive to the power switch turn on noise and spikes and practically in most applications, no filtering network is required to sense the current. Finally, this pin is used: – as a protection against over currents (Isense > I) – as a reduction of the peak current during a Pulsed Mode switching phase. The overcurrent propagation delay is reduced by producing a sharp output turn off (high slew rate). This results in an abrupt output turn off in the event of an over current and in the majority of the pulsed mode switching sequense. Demagnetization Section The MC44608 demagnetization detection consists of a comparator designed to compare the VCC winding voltage to a reference that is typically equal to 50mV. This reference is chosen low to increase effectiveness of the demagnetization detection even during start–up. A latch is incorporated to turn the demagnetization block output into a low level as soon as a voltage less than 50 mV is detected, and to keep it in this state until a new pulse is generated on the output. This avoids any ringing on the input signal which may alter the demagnetization detection. For a higher safety, the demagnetization block output is also directly connected to the output, which is disabled during the demagnetization phase. The demagnetization pin is also used for the quick, programmable OVP. In fact, the demagnetization input current is sensed so that the circuit output is latched off when this current is detected as higher than 120 µA. Figure 3. Demagnetization Block & 1 DMG > 24 A Output Current Mirror Oscillator Buffer + – Idemag 50/20 mV >120 A m m Demag RQ S DMG This function can be inhibited by grounding it but in this case, the quick and programmable OVP is also disabled. Oscillator The MC44608 contains a fixed frequency oscillator. It is built around a fixed value capacitor CT succesively charged and discharged by two distinct current sources ICH and IDCH. The window comparator senses the CT voltage value and activates the sources when the voltage is reaching the 2.4V/4V levels. Figure 4. Oscillator Block + – from Demag 4 V 2.4 V & Window OSC ICH comp DMG SDCH IDCH logic block SCH Clock CT The complete demagnetization status DMG is used to inhibit the recharge of the CT capacitor. Thus in case of incomplete transformer demagnetization the next switching cycle is postpone until the DMG signal appears. The oscillator remains at 2.4V corresponding to the sawtooth valley voltage. In this way the SMPS is working in the so called SOPS mode (Self Oscillating Power Supply). In that case the effective switching frequency is variable and no longer depends on the oscillator timing but on the external working conditions (Refer to DMG signal in the Figure 5). |
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