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NCP1126 Datasheet(PDF) 10 Page - ON Semiconductor |
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NCP1126 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 13 page NCP1126, NCP1129 http://onsemi.com 10 latched−state, a permanent current must be injected in the part. If too low of a current, the part de−latches and the converter resumes operation. This current is characterized to 32 mA as a minimum but we recommend including a design margin and select a value around 60 mA. The test is to latch the part and reduce the input voltage until it de−latches. If you de−latch at Vin = 70 V rms for a minimum voltage of 85 V rms, you are fine. If it precociously recovers, you will have to increase the start−up current, unfortunately to the detriment of standby power. The most sensitive configuration is actually that of the half−wave connection proposed in Figure 3. As the current disappears 5 ms for a 10 ms period (50 Hz input source), the latch can potentially open at low line. If you really reduce the start−up current for a low standby power design, you must ensure enough current in the SCR in case of a faulty event. An alternate connection to the above is shown below (Figure 4): Figure 4. The Full−Wave Connection Ensures Latch Current Continuity as well as a X2−Discharge Path In this case, the current is no longer made of 5 ms “holes” and the part can be maintained at a low input voltage. Experiments show that these 2 M W resistor help to maintain the latch down to less than 50 V rms, giving an excellent design margin. Standby power with this approach was also improved compared to Figure 3 solution. Please note that these resistors also ensure the discharge of the X2−capacitor up to a 0.47 mF type. The de−latch of the SCR occurs when a) the injected current in the Vcc pin falls below the minimum stated in the data−sheet (32 mA at room temp) or when the part senses a brown−out recovery. Frequency Foldback The reduction of no−load standby power associated with the need for improving the efficiency, requires a change in the traditional fixed−frequency type of operation. This switcher implements a switching frequency foldback when the feedback voltage passes below a certain level, Vfold, set around 1.5 V. At this point, the oscillator turns into a Voltage−Controlled Oscillator and reduces its switching frequency. The peak current setpoint is following the feedback pin until its level reaches 1 V. Below this value, the peak current freezes to Vfreeze/4 (250 mV or ≈31% of the maximum 0.8 V setpoint) and the only way to further reduce the transmitted power is to diminish the operating frequency down to 26 kHz. This value is reached at a voltage feedback level of 450 mV typically. Below this point, if the output power continues to decrease, the part enters skip cycle for the best noise−free performance in no−load conditions. Figure 5 depicts the adopted scheme for the part. |
Similar Part No. - NCP1126_12 |
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Similar Description - NCP1126_12 |
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