SLUS458I July 2000 – June 2024 UCC28C40 , UCC28C41 , UCC28C42 , UCC28C43 , UCC28C44 , UCC28C45 , UCC38C40 , UCC38C41 , UCC38C42 , UCC38C43 , UCC38C44 , UCC38C45
PRODUCTION DATA
An external series resistor (RCS) senses the current and converts this current into a voltage that becomes the input to the CS pin. The CS pin is the noninverting input to the PWM comparator. The device compares the CS input with a signal proportional to the error amplifier output voltage. The gain of the current sense amplifier is typically 3V/V. The peak ISENSE current is determined using Equation 2
The typical value for VCS is 1V. A small RC filter (RCSF and CCSF) may be required to suppress switch transients caused by the reverse recovery of a secondary side diode or equivalent capacitive loading in addition to parasitic circuit impedances. The time constant of this filter should be considerably less than the switching period of the converter.
Cycle-by-cycle pulse width modulation performed at the PWM comparator essentially compares the error amplifier output to the current sense input. This is not a direct volt-to-volt comparison, as the error amplifier output network incorporates two diodes in series with a resistive divider network before connecting to the PWM comparator. The two-diode drop adds an offset voltage that enables zero duty cycle to be achieved with a low amplifier output. The 2R/R resistive divider facilitates the use of a wider error amplifier output swing that can be more symmetrically centered on the 2.5V noninverting input voltage.
The 1V Zener diode associated with the PWM comparator input from the error amplifier is not an actual diode in the device design, but an indication that the maximum current sense input amplitude is 1V (typical). When this threshold is reached, regardless of the error amplifier output voltage, cycle-by-cycle current limiting occurs, and the output pulse width is terminated within 35ns (typical). The minimum value for this current limit threshold is 0.9V with a 1.1V maximum. In addition to the tolerance of this parameter, the accuracy of the current sense resistor, or current sense circuitry, must be taken into account. It is advised to factor in the worst case of primary and secondary currents when sizing the ratings and worst-case conditions in all power semiconductors and magnetic components.