27:

TIME (4 Ps/DIV)

200 mA/DIV

2V/DIV

20 mV/DIV

AC Coupled

Typically

42:

LP3905

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SNVS374D – JUNE 2006 – REVISED MAY 2013

PFM OPERATION

At very light loads, the converters enters PFM mode and operate with reduced switching frequency and supply

current to maintain high efficiency.

The Bucks will automatically transition into PFM mode when either of two conditions occurs for a duration of 32

or more clock cycles:

A. The inductor current becomes discontinuous.

(1)

Figure 8. Typical PFM Operation

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

PFM mode. The typical peak current in PFM mode is:

(2)

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 ), 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 16µA (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 causing the output voltage to fall below the ‘low2’ PFM

transitions from PWM to PFM mode at ~35mA output current and from PFM to PWM mode at ~85mA , when

=4.5V, PWM to PFM transition happens at ~65mA and PFM to PWM transition happens at ~115mA.

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