Simple Change Improves PFM Boost-Controller Efficiency

Abstract

Modification to the standard PFM boost-controller circuit provides a 5% efficiency improvement at medium and light loads.

A simple modification to the standard application circuit for a high-efficiency PFM boost controller (Figure 1) yields even higher efficiency. By increasing the value of R_SENSE and connecting the output filter capacitor to the current-sense pin (CS) instead of ground, the circuit's current limit is made dependent on load current. The result is lower I²R loss (in the inductor, MOSFET, and output-capacitor ESR), which gives better efficiency for light-to-medium loads.

Figure 1. Connecting the COUT capacitor to CS instead of ground causes load-dependent current limiting, which reduces I2R loss.

Figure 1. Connecting the C_OUT capacitor to CS instead of ground causes load-dependent current limiting, which reduces I²R loss.

Connecting the filter capacitor to CS allows R_SENSE to monitor the inductor current constantly-via the MOSFET during t_ON and via the diode and filter capacitor during t_OFF. During t_ON, the filter-capacitor voltage drives load current in a loop (C_OUT/LOAD/R_SENSE) that opposes the inductor current through R_SENSE. In effect, the CS node subtracts load current from inductor current during this interval. Thus, as load current increases, the higher level of inductor current required to produce 100mV across R_SENSE extends the _ON interval and raises the current limit:

I_LIM = (100mV/R_SENSE) + I_LOAD.

This modification does not affect the quiescent current and requires no additional circuitry, but the voltage waveform at CS couples through C_OUT to the output, increasing the output ripple about 100mV for light to medium loads. To obtain a lower peak current and higher efficiency for light to medium loads, the value of R_SENSE should be increased as necessary to obtain the same current limit at maximum load as that provided by the standard application circuit. Figure 2 shows the effect of a load transient on the inductor current and output ripple, and Figure 3 shows efficiency gains over the standard connection.

Figure 2. An abrupt change in load current (300mA to 1A) causes changes as shown in the inductor current (top trace, 1A/div) and VOUT ripple (ac-coupled bottom trace, 100mV/div).

Figure 2. An abrupt change in load current (300mA to 1A) causes changes as shown in the inductor current (top trace, 1A/div) and V_OUT ripple (ac-coupled bottom trace, 100mV/div).

Figure 3. Efficiency for the Figure 1 circuit is 4-5% better than that of a standard connection.