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.

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

20040905

FIGURE 2. Simplified Functional Diagram

200409A7

FIGURE 3. Typical Circuit Waveforms in PWM Mode

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