Datasheet LTC3775 (Analog Devices) - 10

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