A controller (10) for a
constant frequency current-mode buck-
boost converter, the converter including an
inductor (L), a first switch (SW1, S1) connected between an input terminal (Vin) of the converter and one end of the
inductor (L), a
diode or a switch (D1, S2), connected between the first end of the
inductor and ground, a second
diode or switch (D2, S3) connected between a second end of the inductor and an output terminal (Vout) of the converter, and a second switch (SW2, S4) coupled between the other end of the inductor and ground. A
current sensor is also present for sensing current in the first switch (SW1, S1) as a measure of the inductor (L) current. The controller (10) comprises a feedback circuit for deriving a feedback
signal from the output terminal (Vout) and a waveform generator (31, 32) for generating buck and boost slope compensation ramps (RMP-BUCK, RMP-BOOST) used for improving stability of operation in both buck and boost operating
modes of the converter.
Control logic in the controller (10) generates drive signals during every
clock period to repeatedly open and close the switches at individual duty cycles determined on the basis of the feedback
signal, the sensed current
signal and the slope compensation ramps. The buck and boost slope compensation ramps are mutually offset with respect to one another in time, for example by 50% of a
clock cycle, and the
control logic uses the sensed current signal only while the first switch (SW1, S1) is closed, thus avoiding the need for additional current sensing circuitry.