Datasheet LT1934, LT1934 (Analog Devices) - 7

LT1934/LT1934-1
OPERATION (Refer to Block Diagram)
The LT1934 uses Burst Mode control, combining both low fl ip-fl op when this current reaches 400mA (120mA for quiescent current operation and high switching frequency, the LT1934-1). After the 1.8μs delay of the off-time one- which result in high effi ciency across a wide range of load shot, the cycle repeats. Generally, the LT1934 will reach currents and a small total circuit size. current limit on every cycle—the off time is fi xed and the on time is regulated so that the LT1934 operates at A comparator monitors the voltage at the FB pin of the the correct duty cycle. The 1.8μs off time is lengthened LT1934. If this voltage is higher than the internal 1.25V when the FB pin voltage falls below 0.8V; this foldback reference, the comparator disables the oscillator and power behavior helps control the output current during start-up switch. In this state, only the comparator, reference and and overload. Figure 1 shows several waveforms of an undervoltage lockout circuits are active, and the current into LT1934 producing 3.3V from a 10V input. When the switch the VIN pin is just 12μA. As the load current discharges the is on, the SW pin voltage is at 10V. When the switch is output capacitor, the voltage at the FB pin falls below 1.25V off, the inductor current pulls the SW pin down until it is and the comparator enables the oscillator. The LT1934 clamped near ground by the external catch diode. begins to switch, delivering current to the output capaci- tor. The output voltage rises, and when it overcomes the The switch driver operates from either the input or from feedback comparator’s hysteresis, the oscillator is disabled the BOOST pin. An external capacitor and diode are used and the LT1934 returns to its micropower state. to generate a voltage at the BOOST pin that is higher than the input supply. This allows the driver to fully saturate The oscillator consists of two one-shots and a fl ip-fl op. the bipolar switch for effi cient operation. A rising edge from the off-time one-shot sets the fl ip-fl op, which turns on the internal NPN power switch. The switch If the SHDN pin is grounded, all internal circuits are turned remains on until either the on-time one-shot trips or the off and VIN current reduces to the device leakage current, current limit is reached. A sense resistor and amplifi er typically a few nA. monitor the current through the switch and resets the VOUT 50mV/DIV VSW 10V/DIV ISW 0.5A/DIV ILI 0.5A/DIV 1934 F01a 5μs/DIV
Figure 1. Operating Waveforms of the LT1934 Converting 10V to 3.3V at 180mA (Front Page Schematic)
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