lm2706 National Semiconductor Corporation, lm2706 Datasheet - Page 10

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

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Circuit Operation
PWM Operation
While in PWM (Pulse Width Modulation) mode, the output
voltage is regulated by switching at a constant frequency
and then modulating the energy per cycle to control power to
the load. Energy per cycle is set by modulating the PFET
switch on-time pulse-width to control the peak inductor cur-
rent. This is done by comparing the signal from the current-
sense amplifier with a slope compensated error signal from
the voltage-feedback error amplifier. At the beginning of
each cycle, the clock turns on the PFET switch, causing the
inductor current to ramp up. When the current sense signal
ramps past the error amplifier signal, the PWM comparator
turns off the PFET switch and turns on the NFET synchro-
nous rectifier, ending the first part of the cycle. If an increase
in load pulls the output down, the error amplifier output
increases, which allows the inductor current to ramp higher
before the comparator turns off the PFET. This increases the
average current sent to the output and adjusts for the in-
crease in the load.
Before going to the PWM comparator, the error signal is
summed with a slope compensation ramp from the oscillator
for stability of the current feedback loop. During the second
part of the cycle, a zero crossing detector turns off the NFET
synchronous rectifier if the inductor current ramps to zero.
The minimum on time of PFET in PWM mode is 200 ns.
(Continued)
FIGURE 2. Simplified Functional Diagram
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Bypass Operation
The LM2706 contains an internal PFET switch for bypassing
the PWM DC-DC converter during Forced and Automatic
Bypass modes. In Forced Bypass mode, this switch is turned
on to power the PA directly from the battery for maximum RF
output power. Automatic Bypass mode turns on the bypass
switch when the input voltage approaches the programmed
output voltage so that the RF PA can continue to operate
with good linearity at high power levels when the battery
voltage is low. When the part operates in the Forced or
FIGURE 3. Typical Circuit Waveforms in PWM Mode
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