Datasheet

TPS63036

www.ti.com

SLVSB76 –AUGUST 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. A resistive voltage divider must be connected to that pin. 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. 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

step down operation to boost operation and back as required by the configuration. 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 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.

For the remaining 2 switches, one is kept permanently on and the other is kept permanently off, thus causing no

switching 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 1 shows the control loop.

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

Gmv defines the average inductor current. The inductor current is reconstructed measuring the current through

the high side buck MOSFET. This current corresponds exactly to the inductor current in boost mode. In buck

mode the current is measured during the on time of the same MOSFET. During the off time the current is

reconstructed internally starting from the peak value reached at the end of the on time cycle. The average

current is then 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, on the other hand 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

has been 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.

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