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
Ipfm 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 Vout
towards
Low2 PFM
Threshold
Low2 PFM Threshold,
switch back to PWMmode
Load current
increases
Low2 PFM Threshold
Vout
I
PFM Peak
=
V
IN
64:
117 mA +
(typ)
I
MODE
<
V
IN
50:
26 mA +
(typ)
LM3670
SNVS250E –NOVEMBER 2004–REVISED FEBRUARY 2013 www.ti.com
PFM Operation
At very light load, the converter enters PFM mode and operates with reduced switching frequency and supply
current to maintain high efficiency.
The part 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
B. The peak PMOS switch current drops below the I
MODE
level:
(3)
During PFM operation, the converter positions the output voltage slightly higher than the nominal output voltage
in PWM operation, allowing additional headroom for voltage drop during a load transient from light to heavy load.
The PFM comparator senses the output voltage via the feedback pin and control the switching of the output
FETs such that the output voltage ramps between 0.8% and 1.6% (typ) 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 exceeds the ‘high’ PFM threshold or the peak current exceeds the I
PFM
level
set for PFM mode. The peak current in PFM mode is:
(4)
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 24), 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 24) causing the output voltage to fall below the
‘low2’ PFM threshold, the part automatically transitions into fixed-frequency PWM mode.
Figure 24. Operation in PFM Mode and Transition to PWM Mode
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