7.3.1 Operation

If designed as a boost converter, the TPS55330 device regulates the output with current-mode pulse-width-modulation (PWM) control. The PWM-control circuitry turns on the switch at the beginning of each oscillator clock cycle. The input voltage is applied across the inductor and stores the energy as inductor current ramps up. During this portion of the switching cycle, the load current is provided by the output capacitor. When the inductor current reaches a threshold level set by the error amplifier output, the power switch turns off and the external Schottky diode is forward biased to allow the inductor current to flow to the output. The inductor transfers stored energy to replenish the output capacitor and supply the load current. This operation repeats every switching cycle. The duty cycle of the converter is determined by the PWM-control comparator which compares the error amplifier output and the current signal. The oscillator frequency is programmed by the external resistor or synchronized to an external clock signal.

A ramp signal from the oscillator is added to the inductor current ramp to provide slope compensation. Slope compensation is required to avoid subharmonic oscillation that is intrinsic to peak-current mode control at duty cycles higher than 50%. If the inductor value is too small, the internal slope compensation may not be adequate to maintain stability.

The PWM control feedback loop regulates the FB pin to a reference voltage through a transconductance error amplifier. The output of the error amplifier is connected to the COMP pin. An external RC-compensation network connected to the COMP pin is chosen for feedback loop stability and optimum transient response.