lm2706 National Semiconductor Corporation, lm2706 Datasheet - Page 9

• Manufacturer Part Number: lm2706
• Description: Miniature, Variable, Step-down Dc-dc Converter With Bypass For Rf Power Amplifiers
• Manufacturer: National Semiconductor Corporation

Device Information
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
(Continued)
IN
FIGURE 1. Typical Operating Circuit
– V
OUT
)/L, by
9
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.
OUT
20040904
/L. If the inductor current
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