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NCP1338DR2G Datasheet(PDF) 7 Page - ON Semiconductor |
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NCP1338DR2G Datasheet(HTML) 7 Page - ON Semiconductor |
7 / 15 page NCP1338 http://onsemi.com 7 APPLICATION INFORMATION INTRODUCTION The NCP1338 implements a standard current mode architecture where the switch−off time is dictated by the peak current setpoint, whereas the core−reset detection triggers the turn−on event. This component represents the ideal candidate where low part−count is the key parameter, particularly in low−cost ac−dc adapters, consumer electronics, auxiliary supplies, etc. Due to its high−performance, high−voltage technology, the NCP1338 incorporates all the necessary features needed to build a rugged and reliable Switch−Mode Power Supply (SMPS): • Quasi−Resonant Operation: Valley−switching operation is ensured whatever the operating conditions are, due to the internal soxyless circuitry. As a result, there are virtually no primary switch turn−on losses, and no secondary diode recovery losses, and EMI and video noise perturbations are reduced. The converter also stays a first−order system and accordingly eases the feedback loop design. • Dynamic Self−Supply (DSS): Due to its Very High Voltage Integrated Circuit (VHVIC) technology, ON Semiconductor’s NCP1338 allows for a direct pin connection to the high−voltage DC rail. A dynamic current source charges up a capacitor and thus provides a fully independent VCC level. As a result, low power applications will not require any auxiliary winding to supply the controller. In applications where this winding is anyway required (see “Power Dissipation” section in the application note), the DSS will simplify the VCC capacitor selection. • Overcurrent Protection (OCP): When the feedback current is below minimum value, a fault is detected. If this fault is present for more than 80 ms, NCP1338 enters an auto−recovery soft burst mode. All pulses are stopped and the VCC capacitor discharges down to 5.0 V. Then, by monitoring the VCC level, the startup current source is activated ON and OFF to create a burst mode. After the current source being activated twice, the controller tries to restart, with a 4.0 ms soft−start. If the fault has gone, the SMPS resumes operation. If the fault is still there, the burst sequence starts again. The soft−start, together with a minimum frequency clamp, allow to reduce the noise generated in the transformer in short−circuit conditions. • Overvoltage Protection (OVP): By continuously monitoring the VCC voltage level, the NCP1338 stops switching whenever an overvoltage condition is detected. • Brown−Out Detection (BO): By monitoring the level on Pin 1 during normal operation, the controller protects the SMPS against low mains condition. When Pin 1 level falls below 500 mV, the controller stops switching until this level goes back and resumes operation. By adjusting the resistor divider connected between the high input voltage and this pin, start and stop levels are programmable. • Over Power Compensation (OPC): An internal current source injects out of Pin 3 (CS pin) a current proportional to the voltage applied on Pin 1. As this voltage is an image of the input voltage, by inserting a resistor in series with Pin 3, it is possible to create an offset on the current sense signal that will compensate the effect of the input voltage variation. • External Latch Trip Point: By externally forcing a level on Pin 1 (e.g., with a signal coming from a temperature sensor) greater than 3.0 V (but below 5.0 V), it is possible to disable the output of the controller. If the voltage is forced over 5.0 V, the controller is permanently latched−off: to resume normal operation, the VCC voltage should go below 4.0 V, which implies to unplug the SMPS from the mains. • Standby Ability: Under low load conditions, NCP1338 enters a Soft−Skip mode: when the CS setpoint becomes lower than 20% of the maximum peak current, output pulses are stopped, then switching is starting again when FB loop forces a setpoint higher than 25%. As this occurs at low peak current, with Soft−Skip activated, and as the TOFF is clamped, noise−free operation is guaranteed, even with a cheap transformer. |
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