LP3972 datasheet(19/59 Pages) NSC

19 / 59 page

Buck Converter Operation (Continued)

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 IPFM level set for PFM mode. The typical peak

current in PFM mode is: IPFM = 112 mA + V

IN/27

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 3), the PMOS switch

is again turned on and the cycle is repeated 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

21 µA (typ), which allows the part to achieve high efficiencies

under extremely light load conditions. When the output drops

below the ‘low’ PFM threshold, 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 3)

causing the output voltage to fall below the ‘low2’ PFM

threshold, the part will automatically transition into fixed-

frequency PWM mode. Typically when V

IN = 3.6V the part

transitions from PWM to PFM mode at 100 mA output

current .

SHUTDOWN MODE

During shutdown the PFET switch, reference, control and

bias circuitry of the converters are turned off. The NFET

switch will be open in shutdown to discharge the output.

When the converter is enabled, EN, soft start is activated. It

is recommended to disable the converter during the system

power up and undervoltage conditions when the supply is

less than 2.7V.

20207612

FIGURE 2. Typical PFM Operation

20207613

FIGURE 3. Operation in PFM Mode and Transfer to PWM Mode

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