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MC44604 Datasheet(PDF) 15 Page - ON Semiconductor |
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MC44604 Datasheet(HTML) 15 Page - ON Semiconductor |
15 / 22 page MC44604 http://onsemi.com 15 The Sawtooth Generation In the steady state, the oscillator voltage varies between about 1.6 V and 3.6 V. Indeed, the sawtooth is obtained by charging and discharging an external capacitor CT (Pin 10), using two distinct current sources = Icharge and Idischarge. In fact, CT is permanently connected to the charging current source (0.4 Iref) and so, the discharge current source has to be higher than the charge one to be able to decrease the CT voltage. This condition is performed, its value being (2 Iref). Two comparators are used to generate the sawtooth. They compare the CT voltage to the oscillator valley and peak values. The comparison to the low value enables to detect the end of the discharge phase while the comparison to the high value determines when the charge cycle must be stopped. A latch (LDISCH) memorizes the oscillator state. Figure 30. Oscillator 10 CT 1 V Vref 0.4 IREF CVOS PROT COSC HIGH COSC LOW 0 1 IREGUL 1.6 V Q MC44604 COSC REGUL Vosc prot Vosc R S LOSC 3.6 V Q S R DISCH 01 Vdemag out CT< 1.6 V DISCHARGE IDISCHARGE Now, in addition to the charge and discharge cycles, a third state can exist. This phase can be produced when at the end of the discharge phase, the oscillator has to wait for a demagnetization pulse before re−starting. During this delay, the CT voltage must remain equal to the oscillator valley value ( X1.6 V). So, a third regulated current source IREGUL controlled by COSC REGUL, is connected to CT in order to perfectly compensate the (0.4 Iref) current source that permanently supplies CT. On−time is only allowed during the oscillator capacitor charge. So, the maximum duty cycle is 80%. (Note 1) The demagnetization condition is taken into account by a second latch (Losc). (Refer to demagnetization § for further details.) Oscillator Frequency The oscillator frequency can be deducted using the following equations: T charge + C T • D V . I charge T discharge + C T • D V I discharge where: Tcharge is the oscillator charge time DV is the oscillator peak to peak value Icharge is the oscillator charge current and Tdischarge is the oscillator discharge time Idischarge is the oscillator discharge current So, as: fosc = 1 /(Tcharge + Tdischarge) if the REGUL arrangement is not activated, the following equation can be obtained: fosc X 0·395 R ref • C T Demagnetization Block (Note 2) To enable the output, the Losc latch complementary output must be low. Now, this latch reset is activated by the LDISCH output during the discharge phase. So, to restart, the Losc has to be set (refer to Figure 30). To perform this, the demagnetization signal must be low. In a fly−back, a good means to detect the demagnetization consists in using the VCC winding voltage. Indeed this voltage is: − negative during the on−time, − positive during the off−time, − equal to zero for the dead−time with generally a ringing (refer to Figure 31). That is why, the MC44604 demagnetization detection consists of a comparator that can compare the VCC winding voltage to a reference that is typically equal to 65 mV. Note 1. The output is disabled by the signal Vosc prot when VCT is lower than 1 V. (Refer to Figure 29 and Figure 30.) Note 2. The demagnetization detection can be inhibited by connecting pin 8 to the ground. |
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