During PFM operation, the converter positions the output

voltage slightly higher than the nominal output voltage during

PWM operation, allowing additional headroom for voltage

drop during a load transient from light to heavy load. The

PFM comparators sense the output voltage via the feedback

pin and control the switching of the output FETs such that the

output voltage ramps between ~0.6% and ~1.7% above the

nominal PWM output voltage. If the output voltage is below

the ‘high’ PFM comparator threshold, the PMOS power

switch is turned on. It remains on until the output voltage

reaches the ‘high’ PFM threshold or the peak current ex-

ceeds the I

current in PFM mode is: I

Ω .

Once the PMOS power switch is turned off, the NMOS

power switch is turned on until the inductor current ramps to

zero. When the NMOS zero-current condition is detected,

the NMOS power switch is turned off. If the output voltage is

below the ‘high’ PFM comparator threshold (see Figure 7),

the PMOS switch is again turned on and the cycle is re-

peated until the output reaches the desired level. Once the

output reaches the ‘high’ PFM threshold, the NMOS switch is

turned on briefly to ramp the inductor current to zero and

then both output switches are turned off and the part enters

an extremely low power mode. Quiescent supply current

during this ‘sleep’ mode is 16µA (typ), which allows the part

to achieve high efficiencies under extremely light load con-

ditions. When the output drops below the ‘low’ PFM thresh-

old, the cycle repeats to restore the output voltage (average

voltage in pfm mode) to

∼1.15% above the nominal PWM

output voltage.

If the load current should increase during PFM mode (see

Figure 7) causing the output voltage to fall below the ‘low2’

PFM threshold, the part will automatically transition into

fixed-frequency PWM mode. When V

sitions from PWM to PFM mode at ~35mA output current

and from PFM to PWM mode at ~85mA , when V

PWM to PFM transition happens at ~50mA and PFM to

PWM transition happens at ~100mA, when V

to PFM transition happens at ~65mA and PFM to PWM

transition happens at ~115mA.

SHUTDOWN MODE

shutdown mode. During shutdown the PFET switch, NFET

switch, reference, control and bias circuitry of the LM3671

operation. While turning on the device with EN, soft start is

activated. It is recommended to set EN pin low to turn off the

LM3671 during system power up and undervoltage condi-

20108422

FIGURE 6. Typical PFM Operation

20108403

FIGURE 7. Operation in PFM Mode and Transfer to PWM Mode

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