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 + VIN/27Ω. 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 5), 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
16 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
Buck Converter Operation (Continued)
20180712
FIGURE 4. Typical PFM Operation
<
1.15% above the nominal PWM output voltage. If the load
current should increase during PFM mode (see Figure 5)
causing the output voltage to fall below the ‘low2’ PFM
threshold, the part will automatically transition into fixed-
frequency PWM mode. Typically when VIN = 3.6V the part
transitions from PWM to PFM mode at 100 mA output
current.
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
20180713
FIGURE 5. Operation in PFM Mode and Transfer to PWM Mode
15
www.national.com
NSC [ National Semiconductor ]
