Patent TW-200306057-A (TW)
Title: Synchronous buck converter improvements
Inventor: ZHANG JASON
IPC: H02M3/155
Filed: 2003-04-02
Abstract: A synchronous buck converter which provides an improved output current sensing circuit and improved transient behavior during output current stepdown. Also disclosed is a multi-phase synchronous buck converter having improved output current sharing capability, and a multi-phase synchronous buck converter having improved load balancing capability. For improved output current sensing, a sensing circuit includes a sampling switch which operates synchronously on and off with the shunt MOSFET to sample the voltage across the RDS-ON of the shunt MOSFET, and to provide the sampled voltage to a variable gain amplifier. The sensing circuit also includes n RC circuit which implements a low pass filter whereby the output of the variable gain amplifier is substantially independent of the value of the inductor and the magnitude of any time varying component of the signal input to the sampling switch. When the device is packaged an MCM, the current sense circuit gain can be trimmed based on the RDS-ON value. The current sense gain can also be adjusted according to the module temperature by using temperature sensitive devices inside the IC to eliminate the RDS-ON temperature variation, and according to the gate voltage to eliminate the RDS-ON variation due to gate voltage changes. For improved output current sharing, a sensing circuit for each converter stage generates an output signal representative of the output current provided by that converter stage. A master controller provides duty cycle control signals in a predetermined phase relationship for the switching transistors of the individual converter stages according to the difference between the output voltage from the multi-phase converter and a reference signal representing a desired voltage at the output node. A duty cycle trimming for each converter stage modifies the duty cycle control signals from the master controller to equalize as nearly as possible the currents provided by each stage. A current sharing control circuit for each converter stage provides control signals to the duty cycle trimming circuit. This functions to provide a difference signal between the actual current output of a particular stage and either the average of all the stage currents, the smallest stage current, or the largest stage current. For load balancing, a sensing circuit in each converter stage includes a variable gain current sense amplifier that determines the current through the shunt MOSFET to generate an output signal representative of the output current provided by that converter stage. The gains of the current sense amplifiers in each converter stage are preset such that the output signals from all the amplifiers are substantially equal, thereby balancing the power loss in all the converter stages by adjustment of the output currents using varying current sense gains to compensate for variations in the RDS-QN of the shunt MOSFETS. For improved transient response during output current stepdown, a sensing circuit provides a control signal output when the duty cycle for the series MOSFET is zero; and a drive circuit responsive to the control signal output of the sensing circuit turns off the shunt MOSFET The sensing circuit operates by detecting when the error signal which determines the duty cycle for the series MOSFET is of a lower amplitude than the minimum valve of the ramp waveform from which the duty cycle for the series MOSFET is generated.