アプリケーションノート 1170

PFMブーストコンバータに電流モード動作を付加する回路接続


要約: マキシムのPFMブーストコントローラは高効率と低自己消費電流を提供しますが、外部で電流モード信号をフィードバックに加えることで、安定性と出力リップルが向上します。この回路接続は、標準接続と比較すると、追加のICやコンポーネントが不要です。

A control scheme used in certain boost-controller ICs from Maxim (current-limited pulse-frequency modulation, or PFM), achieves high efficiency over a wide range of output current by combining the low quiescent current of PFM with the load-driving capability of pulse-width modulation (PWM). To provide current-mode control as well, simply connect the load, output filter capacitor, and lower feedback resistor to the current-sense pin (CS) instead of ground (Figure 1).

Figure 1. This circuit adds current-mode operation to Maxim's current-limited PFM boost controllers without increasing the quiescent current.
Figure 1. This circuit adds current-mode operation to Maxim's current-limited PFM boost controllers without increasing the quiescent current.

When operating with medium to heavy loads, the Figure 1 circuit exhibits lower output ripple and a more stable inductor current (Figure 2a) than do the standard application circuits represented by typical waveforms in Figure 2b. The improvements gained by these connections have no effect on the quiescent current and require no additional circuitry, but they do require separate input and output grounds as shown, connected only by RSENSE.

Figure 2. The Figure 1 circuit's inductor-current and output-ripple waveforms (a) exhibit better stability and lower ripple amplitude than those of the standard operating configuration (b).
Figure 2. The Figure 1 circuit's inductor-current and output-ripple waveforms (a) exhibit better stability and lower ripple amplitude than those of the standard operating configuration (b).

This circuit achieves current-mode control by constantly monitoring the inductor current through RSENSE: via the field-effect transistor during tON (which depends on the magnitude of VIN), and via the diode and output filter capacitor during tOFF (minimum 2.3µs). Flowing through RSENSE, the inductor current creates a signal at CS that couples through the output filter capacitor and adds to the normal feedback signal at FB. Connecting the load between OUT and CS prevents this CS signal from adding to the ripple at VOUT.

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APP 1170:
アプリケーションノート 1170,AN1170, AN 1170, APP1170, Appnote1170, Appnote 1170