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L4964 Datasheet(PDF) 3 Page - STMicroelectronics |
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L4964 Datasheet(HTML) 3 Page - STMicroelectronics |
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3 / 13 page  CIRCUIT OPERATION (refer to the block diagram) The L4964 is a monolithic stepdown switching regu- lator providing output voltages from 5.1 V to 28 V and delivering 4A. The regulation loop consists of asawtooth oscillator, error amplifier, comparator and the output stage. An error signal is produced by comparing the output voltage with a precise 5.1 V on-chip reference (zener zap trimmed to ± 3 %). This error signal is then compared with the sawtooth signal to generate the fixed frequency pulse width modulated pulses which drive the output stage. The gain and fre- quency stability of the loop can be ajusted by an ex- ternal RC network connected to pin 9. Closing the loop directly gives an output voltage of 5.1 V. Higher voltages are obtained by inserting a voltage divider. Output overcurrents at switch on are prevented by the soft start function. The error amplifier output is initially clamped by the external capacitor Css and al- lowed to rise, linearly, as this capacitor is charged by a constant current source. Output overload protection is provided in the form of a current limiter. The load current is sensed by an internal metal resistor connected to a comparator. When the load current exceeds a preset threshold this comparator sets a flip flop which disables the output stage and discharges the soft start capacitor. A second comparator resets the flip flop when the voltage across the soft start capacitor has fallen to 0.4 V. The output stage is thus re-enable and the outputvoltage rises undercontro of the soft startnet- work. If the overload condition is still present the limiter will trigger again when the thershold current is reached. The average short circuit current is lim- ited to a safe value by the dead time introduced by the soft start network. The reset circuit generates an output signal when the supply voltage exceeds a threshold pro- grammed by an external divider. The reset signal is generated with a delay time programmed by an ex- ternal capacitor. When the supply falls below the threshold the reset output goes low immediately. The reset output is an open collector. A TTL - level input is provided for applications such as remote on/off control. This input is activated by high level and disables circuit operation. After an in- hibit the L4964 restarts under control of the soft start network. The thermal overload circuit disables circuit opera- tion when the junction temperature reaches about 150 and has hysteresis to prevent unstable condi- tions. Figure 1 : Reset Output Waveforms L4964 3/13 |
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