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LT1577 Datasheet(PDF) 11 Page - Linear Technology |
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LT1577 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 20 page  11 LT1575/LT1577 APPLICATIONS INFORMATION Because the MOSFET pass transistor is connected as a source follower, the power path gain is much more pre- dictable than designs that employ a discrete PNP transis- tor as the pass device. This is due to the significant production variations encountered with PNP Beta. MOSFETs are also very high speed devices which enhance the ability to produce a stable wide bandwidth control loop. An additional advantage of the follower topology is inherently good line rejection. Input supply disturbances do not propagate through to the output. The feedback loop for a regulator circuit is completed by providing an error signal to the FB pin in the adjustable voltage version and the OUT pin in the fixed voltage version. In both cases, a resistor divider network senses the output voltage and sets the regulated DC bias point. In general, the LT1575 regulator feedback loop permits a loop crossover fre- quency on the order of 1MHz while maintaining good phase and gain margins. This unity-gain frequency is a factor of 20 to 30 times the bandwidth of currently implemented regulator solutions for microprocessor power supplies. This significant performance benefit is what permits the elimination of all bulk output capacitance. Several other unique features are included in the design that increase its functionality and robustness. These func- tions comprise the remainder of the block diagram. A high side sense, current limit amplifier provides active current limiting for the regulator. The current limit ampli- fier uses an external low value shunt resistor connected in series with the external MOSFET’s drain. This resistor can be a discrete shunt resistor or can be manufactured from a Kelvin-sensed section of “free” PC board trace. All load current flows through the MOSFET drain and thus, through the sense resistor. The advantage of using high side current sensing in this topology is that the MOSFET’s gain and the main feedback loop’s gain remain unaffected. The sense resistor develops a voltage equal to IOUT(RSENSE). The current limit amplifier’s 50mV threshold voltage is a good compromise between power dissipation in the sense resistor, dropout voltage impact and noise immunity. Current limit activates when the sense resistor voltage equals the 50mV threshold. Two events occur when current limit activates: the first is that the current limit amplifier drives Q2 in the block Reference voltage accuracy for the adjustable version and output voltage accuracy for the fixed voltage versions are specified as ±0.6% at room temperature and as ±1% over the full operating temperature range. This places the LT1575/LT1577 family among a select group of regulators with a very tightly specified output voltage tolerance. The accurate 1.21V reference is tied to the noninverting input of the main error amplifier in the feedback control loop. The error amplifier consists of a single high gain gm stage with a transconductance equal to 15 millimhos. The inverting terminal is brought out as the FB pin in the adjustable voltage version and as the OUT pin in fixed voltage versions. The gm stage provides differential-to- single ended conversion at the COMP pin. The output impedance of the gm stage is about 1MΩ and thus, 84dB of typical DC error amplifier open-loop gain is realized along with a typical 75MHz uncompensated unity-gain crossover frequency. Note that the overall feedback loop’s DC gain decreases from the gain provided by the error amplifier by the attenuation factor in the resistor divider network which sets the DC output voltage. These attenuation factors are already built into the Open-Loop Voltage Gain specifications for the LT1575 fixed voltage versions in the Electrical Characteristics table to simplify user calculations. External access to the high impedance gain node of the error amplifier permits typical loop compensation to be accomplished with a series RC network to ground. A high speed, high current output stage buffers the COMP node and drives up to 5000pF of “effective” MOSFET gate capacitance with almost no change in load transient per- formance. The output stage delivers up to 50mA peak when slewing the MOSFET gate in response to load current transients. The typical output impedance of the GATE pin is typically 2 Ω. This pushes the pole due to the error amplifier output impedance and the MOSFET input capacitance well beyond the loop crossover frequency. If the capacitance of the MOSFET used is less than 1500pF, it may be necessary to add a small value series gate resistor of 2 Ω to 10Ω. This gate resistor helps damp the LC resonance created by the MOSFET gate’s lead induc- tance and input capacitance. In addition, the pole formed by this resistance and the MOSFET input capacitance can be fine tuned. |
Similar Part No. - LT1577_15 |
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Similar Description - LT1577_15 |
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