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L4962 Datasheet(PDF) 5 Page - STMicroelectronics |
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L4962 Datasheet(HTML) 5 Page - STMicroelectronics |
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5 / 15 page  CIRCUIT OPERATION (refer to the block diagram) The L4962 is a monolithic stepdownswitching regu- lator providing output voltages from 5.1V to 40V and delivering 1.5A. The regulation loop consists of a sawtooth oscilla- tor, error amplifier, comparator and the output stage. An error signal is produced by comparing the output voltage with a precise 5.1V on-chip refer- ence (zener zap trimmed to ± 2%). This error signal is then comparedwith the sawtooth signal to generate the fixed frequency pulse width modulated pulses which drive the output stage. The gain and frequency stability of the loop can be adjusted by an external RC network connected to pin 11. Closing the loop directly gives an output voltage of 5.1V. 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 allowed to rise, linearly, as this capacitor is charged by a constant current source. Output overload pro- tection 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 com- parator resets the flip flop when the voltage across the soft start capacitor has fallen to 0.4V. The output stage is thus re-enabled and the output voltage rises under control of the soft start network. If the overload condition is still present the limiter will trigger again when the threshold current is reached. The average short circuit current is limited to a safe value by the dead time introduced by the soft start network. The thermal overload circuit dis- ables circuit operation when the junction tempera- ture reaches about 150 °C and has hysteresis to prevent unstable conditions. Figure 1. Soft start waveforms Figure 2. Current limiter waveforms 5/15 L4962 |
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