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LT1534CS Datasheet(PDF) 7 Page - Linear Technology |
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LT1534CS Datasheet(HTML) 7 Page - Linear Technology |
7 / 16 page 7 LT1534/LT1534-1 BLOCK DIAGRA OPERATIO In noise sensitive applications, switching regulators tend to be ruled out as a power supply option due to their propensity for generating unwanted noise. When switch- ing supplies are required due to efficiency or input/output voltage constraints, great pains must be taken to work around the noise generated by a typical supply. These steps may include precise synchronization of the power supply oscillator to an external clock, synchronizing the rest of the circuit to the power supply oscillator, or halting power supply switching during noise sensitive operations. The LT1534 greatly simplifies the task of eliminating supply noise by enabling the design of an inherently low noise switching regulator power supply. The LT1534 is a fixed frequency, current mode switching regulator with unique circuitry to control the voltage and current slew rates of the output switch. Slew control capability provides much greater control over power sup- ply components that can create conducted and radiated electromagnetic interference. The current mode control provides excellent AC and DC line regulation and simplifies loop compensation. Current Mode Control A switching cycle begins with an oscillator discharge pulse which resets the RS flip-flop, turning on the output driver (refer to Block Diagram). The switch current is sensed across an internal resistor and the resulting voltage is amplified and compared to the output of the error amplifier (VC pin). The driver is turned off once the output of the current sense amplifier exceeds the voltage on the VC pin. Internal slope compensation is provided to ensure stabil- ity under high duty cycle conditions. Output regulation is obtained using the error amp to set the switch current trip point. The error amp is a transcon- ductance amplifier that integrates the difference between the feedback output voltage and an internal 1.25V refer- ence. The output of the error amp adjusts the switch current trip point to provide the required load current at the desired regulated output voltage. This method of controlling current rather than voltage provides faster input transient response, cycle by cycle current limiting for better output switch protection and greater ease in compensating the feedback loop. The VC pin serves three different purposes. It is used for loop compensation, current limit adjustment and soft starting. During normal operation the VC voltage will be between 0.2V and 1.33V. An external clamp may be used for lowering the current limit. A capacitor coupled to an external clamp can be used for soft starting. + VC NFB 100k 50k FB RT CT SYNC GND 1534 BD RVSL RCSL OSCILLATOR SQ FF R INTERNAL VCC SHDN VIN PGND COL COL OUTPUT DRIVER – + NEGATIVE FEEDBACK AMP – + gm ERROR AMP 1.25V COMP LDO REGULATOR SLEW CONTROL |
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