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L296 Datasheet(PDF) 3 Page - STMicroelectronics |
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L296 Datasheet(HTML) 3 Page - STMicroelectronics |
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3 / 22 page  CIRCUIT OPERATION (refer to the block diagram) The L296 and L296P are monolithic stepdown switching regulators providing output voltages from 5.1V to 40V and delivering 4A. The regulationloop consists of a sawtoothoscillator, error amplifier, comparatorand the outputstage. An error signal is produced by comparing the output voltage with a precise 5.1V on-chipreference(zener zap trimmed to ± 2 %). This error signalis thencom- pared with the sawtooth signal to generate the fixed frequencypulse width modulatedpulseswhich drive the output stage.The gain and frequencystability of the loop can be adjusted by an externalRC network connectedto pin9. Closing the loop directlygives an outputvoltageof 5.1V.Higher voltagesare 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. Outputoverloadprotection 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 outputstage and discharges the soft start capacitor. A second comparator resets the flip flop when the voltage across the soft start capacitor has fallen to 0.4V. 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 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. The scrowbar circuit senses the output voltage and the crowbar output can provide a current of 100mA to switch on an externalSCR. This SCR is triggered when the output voltage exceeds the nominal by 20%. There is no internal connection between the outputand crowbar sense input thereforethe crow- bar can monitor either the input or the output. A TTL - level inhibit input is provided for applications such as remote on/offcontrol. This input is activated by high logic level and disables circuit operation. Af- ter an inhibit the L296 restarts under control of the soft start network. The thermal overload circuit disables circuit opera- tion when the junction temperature reaches about 150 °C and has hysteresis to prevent unstablecon- ditions. Figure 1 : Reset Output Waveforms L296 - L296P 3/22 |
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