Datasheet

TPS63060

TPS63061

www.ti.com

SLVSA92A –DECEMBER 2011–REVISED FEBRUARY 2012

DETAILED DESCRIPTION

The controller circuit of the device is based on an average current mode topology. The controller also uses input

and output voltage feedforward. Changes of input and output voltage are monitored and immediately can change

the duty cycle in the modulator to achieve a fast response to those errors. The voltage error amplifier gets its

feedback input from the FB pin. At adjustable output voltages, a resistive voltage divider must be connected to

that pin. At fixed output voltages, FB must be connected to the output voltage to directly sense the voltage. Fixed

output voltage versions use a trimmed internal resistive divider. The feedback voltage will be compared with the

internal reference voltage to generate a stable and accurate output voltage.

The device uses 4 internal N-channel MOSFETs to maintain synchronous power conversion at all possible

operating conditions. This enables the device to keep high efficiency over a wide input voltage and output power

range. To avoid ground shift problems due to the high currents in the switches, two separate ground pins GND

and PGND are used. The reference for all control functions is the GND pin. The power switches are connected to

PGND. Both grounds must be connected on the PCB at only one point, ideally, close to the GND pin. Due to the

4-switch topology, the load is always disconnected from the input during shutdown of the converter. To protect

the device from overheating an internal temperature sensor is implemented.

Buck-Boost Operation

To regulate the output voltage at all possible input voltage conditions, the device automatically switches from

buck operation to boost operation and back as required. It always uses one active switch, one rectifying switch,

one switch permanently on, and one switch permanently off. Therefore, it operates as a step down converter

(buck) when the input voltage is higher than the output voltage, and as a boost converter when the input voltage

is lower than the output voltage. There is no mode of operation in which all 4 switches are permanently

switching. Controlling the switches this way allows the converter to maintain high efficiency at the most important

point of operation, when the input voltage is close to the output voltage. The RMS current through the switches

and the inductor is kept at a minimum, to minimize switching and conduction losses.

Control Loop Description

The average inductor current is regulated by a fast current regulator loop which is controlled by a voltage control

loop. Figure 28 shows the control loop.

The non inverting input of the transconductance amplifier Gmc can be assumed to be constant. The output of

Gmv defines the average inductor current. The current through resistor RS, which represents the actual inductor

current, is compared to the desired value and the difference, or current error, is amplified and compared to the

sawtooth ramp of either the Buck or the Boost.

The Buck-Boost Overlap Control™ makes sure that the classical buck-boost function, which would cause two

switches to be on every half a cycle, is avoided. Thanks to this block whenever all switches becomes active

during one clock cycle, the two ramps are shifted away from each other. However, when there is no switching

activities because there is a gap between the ramps, the ramps are moved closer together. As a result the

number of classical buck-boost cycles or no switching is reduced to a minimum and high efficiency values are

achieved.

Slope compensation is not required to avoid subharmonic oscillation which are otherwise observed when working

with peak current mode control with D>0.5.

Nevertheless the amplified inductor current downslope at one input of the PWM comparator must not exceed the

oscillator ramp slope at the other comparator input. This purpose is reached limiting the gain of the current

amplifier.

Product Folder Link(s): TPS63060 TPS63061