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 RSENSE 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.
Connecting the filter capacitor to CS allows RSENSE to monitor the inductor current constantly-via the MOSFET during tON and via the diode and filter capacitor during tOFF. During tON, the filter-capacitor voltage drives load current in a loop (COUT/LOAD/RSENSE) that opposes the inductor current through RSENSE. 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 RSENSE extends the ON interval and raises the current limit:
ILIM = (100mV/RSENSE) + ILOAD.
This modification does not affect the quiescent current and requires no additional circuitry, but the voltage waveform at CS couples through COUT 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 RSENSE 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 3. Efficiency for the Figure 1 circuit is 4-5% better than that of a standard connection.
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