Datasheet LT1534 (Analog Devices) - 7

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