# Simple Change Improves PFM Boost-Controller Efficiency

### 要約

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 I^{2}R loss (in the inductor, MOSFET, and output-capacitor ESR), which gives better efficiency for light-to-medium loads.

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.