Circuit Operation

Referring to Figures 1, 2, 3, the LM2706 operates as follows.

During the first part of each switching cycle, the control block

in the LM2706 turns on the internal PFET switch. This allows

current to flow from the input through the inductor to the

output filter capacitor and load. The inductor limits the cur-

rent to a ramp with a slope of around (V

storing energy in a magnetic field. During the second part of

each cycle, the controller turns the PFET switch off, blocking

current flow from the input, and then turns the NFET syn-

chronous rectifier on. In response, the inductor’s magnetic

field collapses, generating a voltage that forces current from

ground through the synchronous rectifier to the output filter

capacitor and load. As the stored energy is transferred back

into the circuit and depleted, the inductor current ramps

down with a slope around V

reaches zero before the next cycle, the synchronous rectifier

is turned off to prevent current reversal. The output filter

capacitor stores charge when the inductor current is high,

and releases it when low, smoothing the voltage across the

load.

The output voltage is regulated by modulating the PFET

switch on time to control the average current sent to the load.

The effect is identical to sending a duty-cycle modulated

rectangular wave formed by the switch and synchronous

rectifier at SW to a low-pass filter formed by the inductor and

output filter capacitor. The output voltage is equal to the

average voltage at the SW pin.

20040904

FIGURE 1. Typical Operating Circuit

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