LM26 datasheet(9/16 Pages) NSC

9 / 16 page

Device Information (Continued)

CIRCUIT OPERATION

Referring to Figure 4, Figure 5, Figure 6 and Figure 7,

the LM2614 operates as follows. During the first part of each

switching cycle, the control block in the LM2614 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 current to a ramp with a slope of

IN–VOUT)/L, by 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 synchronous 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 of V

OUT/L. If the inductor

current 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.

20036706

FIGURE 4. Typical Operating Circuit

20036707

FIGURE 5. Simplified Functional Diagram

LM26 Scheda tecnica(PDF) 9 Page - National Semiconductor (TI)