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UC1848 bảng dữ liệu(PDF) 1 Page - Texas Instruments |
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1 / 9 page UC1848 UC2848 UC3848 4/96 FEATURES • Practical Primary Side Control of Isolated Power Supplies with DC Control of Secondary Side Current • Accurate Programmable Maximum Duty Cycle Clamp • Maximum Volt-Second Product Clamp to Prevent Core Saturation • Practical Operation Up to 1MHz • High Current (2A Pk) Totem Pole Output Driver • Wide Bandwidth (8MHz) Current Error Amplifier • Under Voltage Lockout Monitors VCC, VIN and VREF • Output Active Low During UVLO • Low Startup Current (500µA) • Precision 5V Reference (1%) The UC3848 family of PWM control ICs makes primary side average current mode control practical for isolated switching converters. Average current mode control in- sures that both cycle by cycle peak switch current and maximum average inductor current are well defined and will not run away in a short circuit situation. The UC3848 can be used to control a wide variety of converter topolo- gies. In addition to the basic functions required for pulse width modulation, the UC3848 implements a patented tech- nique of sensing secondary current in an isolated buck derived converter from the primary side. A current wave- form synthesizer monitors switch current and simulates the inductor current down slope so that the complete cur- rent waveform can be constructed on the primary side without actual secondary side measurement. This infor- mation on the primary side allows for full DC control of output current. The UC3848 circuitry includes a precision reference, a wide bandwidth error amplifier for average current con- trol, an oscillator to generate the system clock, latching PWM comparator and logic circuits, and a high current output driver. The current error amplifier easily interfaces with an optoisolator from a secondary side voltage sens- ing circuit. A full featured undervoltage lockout (UVLO) circuit is con- tained in the UC3848. UVLO monitors the supply voltage to the controller (VCC), the reference voltage (VREF), and the input line voltage (VIN). All three must be good before soft start commences. If either VCC or VIN is low, the supply current required by the chip is only 500 µA and the output is actively held low. Two on board protection features set controlled limits to prevent transformer core saturation. Input voltage is mon- itored and pulse width is constrained to limit the maxi- mum volt-second product applied to the transformer. A unique patented technique limits maximum duty cycle within 3% of a user programmed value. These two features allow for more optimal use of trans- formers and switches, resulting in reduced system size and cost. Patents embodied in the UC3848 belong to Lambda Electronics Incorporated and are licensed for use in ap- plications employing these devices. DESCRIPTION BLOCK DIAGRAM Average Current Mode PWM Controller UDG-93003-1 |
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Mô tả tương tự - UC1848 |
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