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NCP1271 Datasheet(PDF) 10 Page - ON Semiconductor |
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NCP1271 Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 19 page ![]() NCP1271 http://onsemi.com 10 OPERATING DESCRIPTION Introduction The NCP1271 represents a new generation of the fixed−frequency PWM current−mode flyback controllers from ON Semiconductor. The device features integrated high−voltage startup and excellent standby performance. The proprietary Soft−Skip Mode achieves extremely low−standby power consumption while keeping power supply acoustic noise to a minimum. The key features of the NCP1271 are as follows: • Timer−Based Fault Detection: In the event that an abnormally large load is applied to the output for more than 130 ms, the controller will safely shut the application down. This allows accurate overload (OL) or short−circuit (SC) detection which is not dependent on the auxiliary winding. • Soft−Skip Mode: This proprietary feature of the NCP1271 minimizes the standby low−frequency acoustic noise by ramping the peak current envelope whenever skip is activated. • Adjustable Skip Threshold: This feature allows the power level at which the application enters skip to be fully adjusted. Thus, the standby power for various applications can be optimized. The default skip level is 1.2 V (40% of the maximum peak current). • 500 V High−Voltage Startup Capability: This AC−DC application friendly feature eliminates the need for an external startup biasing circuit, minimizes the standby power loss, and saves printed circuit board (PCB) space. • Dual High−Voltage Startup−Current Levels: The NCP1271 uniquely provides the ability to reduce the startup current supply when Vcc is low. This prevents damage if Vcc is ever shorted to ground. After Vcc rises above approximately 600 mV, the startup current increases to its full value and rapidly charges the Vcc capacitor. • Latched Protection: The NCP1271 provides a pin, which if pulled high, places the part in a latched off mode. Therefore, overvoltage (OVP) and overtemperature (OTP) protection can be easily implemented. A noise filter is provided on this function to reduce the chances of falsely triggering the latch. The latch is released when Vcc is cycled below 4 V. • Non−Latched Protection/ Shutdown Option: By pulling the feedback pin below the skip threshold level, a non−latching shutdown mode can be easily implemented. • 4.0 ms Soft−Start: The soft start feature slowly ramps up the drive duty cycle at startup. This forces the primary current to also ramp up slowly and dramatically reduces the stress on power components during startup. • Current−Mode Operation: The NCP1271 uses current−mode control which provides better transient response than voltage−mode control. Current−mode control also inherently limits the cycle−by−cycle primary current. • Compensation Ramp: A drawback of current−mode regulation is that the circuit may become unstable when the operating duty cycle is too high. The NCP1271 offers an adjustable compensation ramp to solve this instability. • 80% Maximum Duty Cycle Protection: This feature limits the maximum on time of the drive to protect the power MOSFET from being continuously on. • Frequency Jittering: Frequency jittering softens the EMI signature by spreading out peak energy within a band +/− 7.5% from the center frequency. • Switching Frequency Options: The NCP1271 is available in either 65 kHz or 100 kHz fixed frequency options. Depending on the application, the designer can pick the right device to help reduce magnetic switching loss or improve the EMI signature before reaching the 150 kHz starting point for more restrictive EMI test limits. NCP1271 Operating Conditions There are 5 possible operating conditions for the NCP1271: 1. Normal Operation – When VCC is above VCC(off) (9.1 V typical) and the feedback pin voltage (VFB) is within the normal operation range (i.e.,VFB < 3.0 V), the NCP1271 operates as a fixed−frequency current−mode PWM controller. 2. Standby Operation (or Skip−Cycle Operation) When the load current drops, the compensation network responds by reducing the primary peak current. When the peak current reaches the skip peak current level, the NCP1271 enters Soft−Skip operation to reduce the power consumption. This Soft−Skip feature offers a modified peak current envelope and hence also reduces the risk of audible noise. In the event of a sudden load increase, the transient load detector (TLD) disables Soft−Skip and applies maximum power to bring the output into regulation as fast as possible. 3. Fault Operation – When no feedback signal is received for 130 ms or when VCC drops below VCC(off) (9.1 V typical), the NCP1271 recognizes it as a fault condition. In this fault mode, the Vcc voltage is forced to go through two cycles of slowly discharging and charging. This is known as a “double hiccup.” The double hiccup insures that ample time is allowed between restarts to prevent overheating of the power devices. If the fault is |
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