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A7986ATR 数据表(PDF) 8 Page - STMicroelectronics |
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A7986ATR 数据表(HTML) 8 Page - STMicroelectronics |
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8 / 42 page  Functional description A7986A 8/42 DocID022801 Rev 5 5 Functional description The A7986A is based on a “voltage mode”, constant frequency control. The output voltage VOUT is sensed by the feedback pin (FB) compared to an internal reference (0.6 V) providing an error signal that, compared to a fixed frequency sawtooth, controls the on and off time of the power switch. The main internal blocks are shown in the block diagram in Figure 3. They are: • A fully integrated oscillator that provides sawtooth to modulate the duty cycle and the synchronization signal. Its switching frequency can be adjusted by an external resistor. The voltage and frequency feed-forward are implemented • The soft-start circuitry to limit inrush current during the startup phase • The voltage mode error amplifier • The pulse width modulator and the relative logic circuitry necessary to drive the internal power switch • The high-side driver for embedded P-channel power MOSFET switch • The peak current limit sensing block, to handle overload and short-circuit conditions • A voltage regulator and internal reference. It supplies internal circuitry and provides a fixed internal reference • A voltage monitor circuitry (UVLO) that checks the input and internal voltages • A thermal shutdown block, to prevent thermal run-away. Figure 3. Block diagram PEAK CURRENT LIMIT OSCILLATOR S R Q THERMAL SHUTDOWN SOFT- START EN TRIMMING UVLO 0.6V REGULATOR & BANDGAP 1.254V 3.3V SYNCH & PHASE SHIFT EN FB COMP FSW GND SYNCH OUT VCC DRIVER E/A PWM PEAK CURRENT LIMIT OSCILLATOR S R Q THERMAL SHUTDOWN SOFT- START EN TRIMMING UVLO UVLO 0.6V REGULATOR & BANDGAP REGULATOR & BANDGAP 1.254V 3.3V SYNCH & PHASE SHIFT EN FB COMP FSW GND SYNCH OUT VCC DRIVER E/A PWM |
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