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LTC3775IUDTRPBF Datasheet(PDF) 10 Page - Linear Technology |
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LTC3775IUDTRPBF Datasheet(HTML) 10 Page - Linear Technology |
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10 / 32 page  LTC3775 10 3775f APPLICATIONS INFORMATION Operation (Refer to Block Diagram) The LTC3775 is a constant frequency, voltage mode con- troller for DC/DC step-down converters. It is designed to be used in a synchronous switching architecture with two external N-channel MOSFETs. For circuit operation, please refer to the Block Diagram. The LTC3775 uses voltage mode control in which the duty cycle is controlled directly by the error amplifier output. The error amplifier adjusts the voltage at the COMP pin by comparing the VFB pin with the 0.6V internal refer- ence. When the load current increases, it causes a drop in the feedback voltage relative to the reference. The COMP voltage then rises, increasing the duty cycle until the LTC3775 output feedback voltage again matches the reference voltage. In normal operation, the top MOSFET is turned on when the PWM comparator changes state and is turned off by the internal oscillator. The PWM comparator maintains the proper duty cycle by comparing the error amplifier output (after being “compensated” by the line feedfor- ward multiplier) to a sawtooth waveform generated by the oscillator. When the top MOSFET is turned off, the bottom MOSFET is turned on until the next cycle begins, or if pulse-skipping mode operation is enabled, until the inductor current reverses as determined by the reverse current comparator. Feedback Control The LTC3775 senses the output voltage at VOUT with an internal feedback op amp (see Block Diagram). This is a true op amp with a low impedance output, 80dB of open- loop gain and a 25MHz gain-bandwidth product. The positive input is connected to an internal 0.6V reference, while the negative input is connected to the FB pin. The output is connected to COMP, which is in turn connected to the line feedforward circuit and from there to the PWM generator. At steady state, as shown in the Block Diagram, the output of the switching regulator is given the following equation VV R R OUT REF A B =+ ⎛ ⎝⎜ ⎞ ⎠⎟ •1 Unlike many regulators that use a transconductance (gm) amplifier, the LTC3775 is designed to use an inverting summing amplifier topology with the FB pin configured as a virtual ground. This allows the feedback gain to be tightly controlled by external components. In addition, the voltage feedback amplifier allows flexibility in choosing pole and zero locations. In particular, it allows the use of “Type 3” compensation, which provides a phase boost at the LC pole frequency and significantly improves the control loop phase margin. In a typical LTC3775 circuit, the feedback loop consists of the line feedforward circuit, the modulator, the external inductor, the output capacitor and the feedback amplifier with its compensation network. All these components affect loop behavior and need to be accounted for in the loop compensation. The modulator consists of the PWM generator, the output MOSFET drivers and the external MOSFETs themselves. The modulator gain varies linearily with the input voltage. The line feedforward circuit com- pensates for this change in gain, and provides a constant gain from the error amplifier output to the inductor input regardless of input voltage. From a feedback loop point of view, the combination of the line feedforward circuit and the modulator looks like a linear voltage transfer function from COMP to the inductor input and has a gain roughly equal to 30V/V. It has fairly benign AC behavior at typical loop compensation frequencies with significant phase shift appearing at half the switching frequency. The external inductor/output capacitor combination makes a more significant contribution to loop behavior. These components cause a second order LC roll-off at the output with 180° phase shift. This roll-off is what filters the PWM waveform, resulting in the desired DC output voltage, but this phase shift causes stability issues in the feedback loop and must be frequency compensated. At higher frequencies, the reactance of the output capacitor approaches its ESR, and the roll-off due to the capacitor stops, leaving –20dB/decade and 90° of phase shift. |
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