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CS51220EDR16 Fiches technique(PDF) 11 Page - ON Semiconductor |
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CS51220EDR16 Fiches technique(HTML) 11 Page - ON Semiconductor |
11 / 16 page CS51220 http://onsemi.com 11 The effect of the soft hiccup can be observed in Figure 9, which shows the output voltage as load increases. The output is maintained at the regulation value of 5.0 V until it goes into current limit. At the point of overcurrent inception (A), the current limit level changes to a lower level (B). The switchback to a lower current limit level can be seen as the bottom curve in Figure 9. 0 Load Current (A) Figure 9. Overcurrent In a 5.0 V Output Converter Using Soft Hiccup 46 6 2 0 2 5 4 3 1 A B A typical overload scenario is shown in Figure 10. The top trace is the voltage on the Soft Start (SS) pin. The initial high discharge rate can be seen transitioning to a 40 ms discharge period. During this period the ISET establishes a lower current limit level. The bottom trace shows the output current. The initial current spike is the output capacitors discharging. The next level around 4.0 A is the short circuit current level set by the ISET current. The output then turns off allowing the current to reduce to a level that does not cause overcurrent pulses. This releases the SS pin to ramp back up. During ramp up, the output is still shorted as noted by the 8.0 A current level. When SS reaches the 2.9 V level, the short is again recognized and ISET is turned back on shifting the short circuit current level. Figure 10. Over−Load Current and Soft Start Waveforms The middle trace is a digitizing ‘scope trace of the current sense line. The scope interprets the voltages as an average voltage. This voltage is actually a narrow duty cycle peak voltage representing the peak current level in the switching transistor. The actual peak voltages can be seen in the Figure 11. The peaks are 0.85 V at full load, reducing to 0.6 V peak at the reduced short circuit level. The 1.1 V peak is the full short circuit current while SS ramps back up. The 0.32 V level is the normal load resistance, while ISET is still on. The 1.0 V surge is created by ramp up into a normal 5.0 A load and followed by the 0.85 V at normal load. Figure 11. Over−Load Current and ISENSE Voltage Peak Detect Setting Oscillator and Synchronization The switching frequency is programmable through a capacitor connected to the CT pin. When the CT pin voltage reaches peak voltage (2.0 V), the internal discharge current discharges the CT capacitor and VO stays low. When the CT voltage declines to valley voltage (0.9 V), the current source toggles to charge current and ramps up the CT pin. This starts a new switching cycle. The duty cycle of the oscillator determines the maximum PWM duty cycle. The switching frequency of the IC can be synchronized to an external frequency presented to the SYNCI pin. When pulses with amplitude over SYNCI input threshold are detected, the CT pin immediately ramps down the external capacitor and the VO pin is forced low. A new switching cycle begins when the CT pin reaches valley voltage. During synchronization, the oscillator charge current is reduced by 80 μA, while discharge current is increased by 80 μA. This effectively slows down the internal oscillator to avoid any race condition with the sync frequency. As a result, the sync frequency can be either higher or lower than the internal oscillator frequency. CS51220 is able to synchronize up to 500 kHz and down to 25% below CT frequency. The maximum duty cycle clamp is raised to 92% in synchronization mode. The original oscillator frequency is restored upon the removal of sync pulses. |
Numéro de pièce similaire - CS51220EDR16 |
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Description similaire - CS51220EDR16 |
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