5.1 Theory of Operation

The integrated high-side switch modulates the input voltage using a controlled duty cycle for output voltage regulation. High efficiency is achieved by using a low-resistance switch, a low forward voltage drop diode, a low equivalent series resistance inductor (DCR) and a capacitor (ESR). When the switch is on, a DC voltage is applied to the inductor (V_IN – V_OUT), resulting in a positive linear ramp of inductor current. When the switch is turned off, the applied inductor voltage is equal to -V_OUT, resulting in a negative linear ramp of inductor current (ignoring the forward voltage drop of the Schottky diode).

In steady-state, continuous inductor current operation, the positive inductor current ramp must equal the negative current ramp in magnitude. While operating in steady state, the switch duty cycle must be equal to the relationship of V_OUT/V_IN for constant output voltage regulation, provided that the inductor current is continuous or never reaches zero. In discontinuous inductor current operation, the steady-state duty cycle is less than V_OUT/V_IN to maintain voltage regulation. The average of the chopped input voltage, or SW node voltage, is equal to the output voltage, while the average of the inductor current is equal to the output current.

For graphical representations of the switching waveform and inductor current in both continuous and discontinuous inductor current modes, see Figure 5-1.

The MCP16367/8/9 features an integrated Peak Current Mode Control architecture, resulting in superior AC regulation while minimizing the number and size of the voltage loop compensation components integrated in the device. Peak Current Mode Control takes a small portion of the inductor current, replicates it and compares this replicated current sense signal with the output voltage of the integrated error amplifier. In practice, the inductor current and the internal switch current are equal during the switch-on time. By adding this peak current sense to the system control, the step-down power train system is reduced from a 2^nd order to a 1^st order. This reduces the system complexity and increases its dynamic performance.