Datasheet

High PFM Threshold
~1.016*Vout
Low1 PFM Threshold
~1.008*Vout
PFM Mode at Light Load
PWM Mode at Moderate to
Heavy Loads
PFET on
until
I
PFM
limit
reached
NFET on
drains
conductor
current
until
I inductor = 0
High PFM
Voltage
Threshold
reached,
go into
sleep mode
Low PFM
Threshold,
turn on
PFET
Current load
increases,
draws V
OUT
towards
Low2 PFM
Threshold
Low2 PFM Threshold,
switch back to PWM mode
Load current
increases
Low2 PFM Threshold
Vout
Z-Axis
Z-
Axis
Z
-
Axis
Z
-
Axis
Z
-
Ax
is
-18
LM26484
SNVS573F –SEPTEMBER 2008–REVISED MARCH 2013 www.ti.com
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 18), 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 less than 30 µA, 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
to ~1.6% above the nominal PWM output voltage.
If the load current should increase during PFM mode (see Figure 18) causing the output voltage to fall below the
‘low2’ PFM threshold, the part will automatically transition into fixed-frequency PWM mode.
During shutdown the PFET switch, reference, control and bias circuitry of the converters are turned off. The
NFET switch will be on in shutdown to discharge the output. When the converter is enabled, soft start is
activated. It is recommended to disable the converter during the system power up and under voltage conditions
when the supply is less than 3.0V.
Figure 18. PFM vs PWM
SOFT START
The soft-start feature allows the power converter to gradually reach the initial steady state operating point, thus
reducing start-up stresses and surges. The two LM26484 buck converters have a soft-start circuit that limits in-
rush current during start-up or the one which ramps up output voltage linearly over about 500 µs. During start-up
the switch current limit is ramped up (100 µs, typ.), depending on the kind of soft-start. Soft start is activated only
if EN goes from logic low to logic high after V
IN
reaches 2.8V.
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