2-Phase, Step-Down Converter IC Configures 12 Phases

Jun 1, 2002 12:00 PM
By Sam Davis, Editor, Power Electronics Technology

Anew phase-locked, fixed-frequency converter operates with 4V to 36V input, providing ±1% output voltage accuracy. The LTC3729 from Linear Technology is a 2-phase, step-down switchmode controller IC that drives synchronous rectifiers consisting of external MOSFETs, as shown in the figure. Using five additional LTC3729s and the associated external MOSFETs, you can configure the combination of these devices to provide up to 12 evenly phased controllers for loads requiring 15A to 200A.

This multiphase technique effectively multiplies the fundamental frequency by the number of channels used, improving transient response while operating each channel at an optimum, efficient frequency. Input capacitance ESR requirements and efficiency losses from the input capacitor are effectively divided by the number of phases used and power loss is proportional to the rms current squared. A two-stage, single-output voltage implementation can reduce input path power loss by 75% and radically reduce the required rms current rating of the input capacitor(s). This technique also improves thermal management by spreading the total power dissipation among all the components in each of the phases.

An internal master oscillator runs at a frequency twelve times that of each controller's frequency. The internally generated clock signal makes it possible to synchronize additional power stages for a multiphase power supply solution feeding a single, high-current output or separate outputs. A phase-locked loop allows the internal oscillator to synchronize with an external source. The output of the internal phase detector is also the dc frequency control input of the oscillator that operates over a 250 kHz to 550 kHz range corresponding to a dc voltage input from 0V to 2.4V.

The LTC3729 drives external N-channel power MOSFETs in a phase-lockable, fixed-frequency architecture. It drives its two output stages out of phase, minimizing rms ripple currents in both input and output capacitors. During normal operation, the top MOSFET turns on each cycle when the IC's oscillator sets its RS latch, and turns off when its main current comparator resets the RS latch. Voltage on the output of its error amplifier (I_TH pin) controls the peak inductor current at which the comparator resets the RS latch. A differential amplifier produces a signal equal to the differential voltage sensed across the output capacitor but re-references it to the internal signal ground (SGND) reference.

A floating bootstrap capacitor biases the top MOSFET drivers. When V_IN decreases to a voltage close to V_OUT, however, the loop may enter dropout and attempt to turn on the top MOSFET continuously. If this occurs, a dropout detector forces the top MOSFET to turn off for about 400 ns every tenth cycle to recharge the bootstrap capacitor.

This IC provides both soft-start and optional timed, short-circuit shutdown. Current foldback limits MOSFET dissipation during short-circuit conditions when the overcurrent latch-off is disabled. OPTI-LOOP compensation optimizes the transient response over a wide range of output capacitance and ESR values. A power good signal indicates when the output is within ±7.5% of the designed set point.

The main control loop is shutdown by pulling the RUN/SS pin low. Releasing RUN/SS allows an internal 1.2µA current source to charge the soft-start capacitor, C_SS, tied to the RUN/SS pin. When C_SS reaches 1.5V, it enables the main control loop with the I_TH voltage clamped at about 30% of its maximum value. As C_SS continues to charge it releases I_TH, allowing normal operation to resume. When the RUN/SS pin is low, all LTC3729 functions shutdown. If V_OUT hasn't reached 70% of its nominal value when C_SS has charged to 4.1V, the IC can invoke an overcurrent latch-off.

Linear Technology, Milpitas, Calif.
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