OB2500NCPA

OB2500N is a high performance offline PSR power switch for low power AC/DC charger and adapter applications. It operates in primary-side sensing and regulation. Consequently, opto-coupler and TL431 could be eliminated. Proprietary Constant Voltage (CV) and Constant Current (CC) control is integrated. In CC control, the current and output power setting can be adjusted externally by the sense resistor Rs at CS pin. In CV control, multi-mode operations are utilized to achieve high performance and high efficiency. In addition, good load regulation is achieved by the built-in cable drop compensation. Device operates in PFM in CC mode at large load condition and it operates in PWM with frequency reduction at light/medium load. The chip consumes very low operation current. It achieves less than 75mW standby power to meet strict standby power standard. OB2500N offers comprehensive protection coverage with auto-recovery feature including Cycle-by-Cycle current limiting, VDD over voltage protection, feedback loop open protection, short circuit protection, built-in leading edge blanking, VDD under voltage lockout (UVLO), OTP etc. OB2500N is offered in SOP7 packages. OB2500N is a cost effective PSR power switch optimized for off-line low power AC/DC applications including battery chargers. It operates in primary side sensing and regulation, thus opto-coupler and TL431 are not required. Proprietary built-in CV and CC control can achieve high precision CC/CV control meeting most charger application requirements. Startup current of OB2500N is designed to be very low so that VDD could be charged up above UVLO threshold level and device starts up quickly. A large value startup resistor can therefore be used to minimize the power loss yet achieve a reliable startup in application. The operating current of OB2500N is as low as 850uA (typical). Good efficiency and les than 75mW standby power is achieved with the low operating current. OB2500N is designed to produce good CC/CV control characteristic. In charger applications, a discharged battery charging starts in the CC portion of the curve until it is nearly full charged and smoothly switches to operate in CV portion of the curve. The CC portion provides output current limiting. In CV operation, the output voltage is regulated through the primary side control. In CC operation mode, OB2500N will regulate the output current constant regardless of the output voltage drop. To support OB2500N series proprietary CC/CV control, system needs to be designed in DCM mode for flyback system. In the DCM flyback converter, the output voltage can be sensed via the auxiliary winding. During MOSFET turn-on time, the load current is supplied from the output filter capacitor, Co, and the current in the primary winding ramps up. When MOSFET turns off, the energy stored in the primary winding is transferred to the secondary side. Via a resistor divider connected between the auxiliary winding and INV (pin 2), the auxiliary voltage is sampled at the middle of the de-magnetization and it is hold until the next sampling. The sampled voltage is compared with reference voltage Vref (typical 2.5V) and the difference is amplified. The error amplifier output reflects the load condition and controls the switching off time to regulate the output voltage, thus constant output voltage can be achieved.

Applications

• Cell Phone Charger
• Digital Cameras Charger
• Small Power Adapter
• Auxiliary Power for PC, TV etc.
• Linear Regulator/RCC Replacement

Key features

• Primary-side sensing and regulation without TL431 and opto-coupler
• High precision constant voltage and current regulation at universal AC input
• Multi-mode PWM/PFM operation for efficiency improving
• Integrated 1A 650V MOSFET
• Good dynamic response
• Programmable CV and CC regulation
• Built-in primary winding inductance compensation
• Programmable cable drop compensation
• No need for control loop compensation
• Audio noise free operation
• Built-in leading edge blanking (LEB)
• Ultra low start-up current and low operating current
• Comprehensive protection coverage with auto-recovery
• On-chip OTP and external OTP
• VDD over voltage protection
• VDD under voltage lockout with hysteresis (UVLO)
• Cycle-by-Cycle current limiting
• Feedback loop open protection
• Output short circuit protection