The NCP1253 is a highly integrated PWM controller capable of delivering a rugged and high performance offline power supply in a tiny TSOP−6 package. With a supply range up to 28 V, the controller hosts a jittered 65 kHz or 100 kHz switching circuitry operated in peak current mode control. When the power on the secondary side starts to decrease, the controller automatically folds back its switching frequency down to a minimum level of 26 kHz. As the power further goes down, the part enters skip cycle while limiting the peak current. To avoid sub harmonic oscillations in CCM operation, adjustable slope compensation is available via the series inclusion of a simple resistor in the current sense signal.
Besides the auto−recovery timer−based short−circuit protection, an Over Voltage Protection on the VCC pin protects the whole circuitry in case of optocoupler destruction or adverse open loop operation.
The NCP1253 implements a standard current mode architecture where the switch−off event is dictated by the peak current setpoint. This component represents the ideal candidate where low part−count and cost effectiveness are the key parameters, particularly in low−cost ac−dc adapters, open−frame power supplies etc. Capitalizing on the NCP1200 series success, the NCP1253 brings all the necessary components normally needed in today modern power supply designs, bringing several enhancements such as a VCC OVP or an adjustable slope compensation signal.
• Current−mode operation with internal ramp compensation: implementing peak current mode control at a fixed 65 kHz or 100 kHz frequency, the NCP1253 offers an internal ramp compensation signal that can easily by summed up to the sensed current. Sub harmonic oscillations can thus be compensated via the inclusion of a simple resistor in series with the current−sense information.
• Low startup current: reaching a low no−load standby power always represents a difficult exercise when the controller draws a significant amount of current during start−up. Thanks to its proprietary architecture, the NCP1253 is guaranteed to draw less than 15 μA maximum, easing the design of low standby power adapters.
• EMI jittering: an internal low−frequency modulation signal varies the pace at which the oscillator frequency is modulated. This helps spreading out energy in conducted noise analysis. To improve the EMI signature at low power levels, the jittering will not be disabled in frequency foldback mode (light load conditions).
• Frequency foldback capability: a continuous flow of pulses is not compatible with no−load/light−load standby power requirements. To excel in this domain, the controller observes the feedback pin and when it reaches a level of 1.5 Vm, the oscillator then starts to reduce its switching frequency as the feedback level continues to decrease. When the feedback pin reaches 1.05 V, the peak current setpoint is internally frozen and the frequency continues to decrease. It can go down to 26 kHz (typical) reached for a feedback level of 350 mV roughly. At this point, if the power continues to drop, the controller enters classi
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