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
Inductor
Current
Ramp up
Inductor
current ramp
down
until
I inductor=0
High PFM
Voltage
Threshold
reached,
go into
Low power
mode, both
switches are off
Low PFM
Threshold,
turn on
Current load
increases,
draws Vout
towards
Low2 PFM
Threshold
Low2 PFM Threshold,
switch back to PWMmode
Load current
increases
Low2 PFM Threshold
Vout
Z-
Ax
is
Z
-
Axis
LM3668
SNVS449M –JUNE 2007–REVISED MAY 2013 www.ti.com
PFM Operation
At very light loads, the converter enters PFM mode and operates with reduced switching frequency and supply
current to maintain high efficiency. The part automatically transitions into PFM mode when either of two following
conditions occur for a duration of 128 or more clock cycles:
A. The inductor current reaches zero.
B. The peak inductor current drops below the I
MODE
level, (Typically I
MODE
< 45 mA + V
IN
/80 Ω ).
In PFM operation, the compensation circuit in the error amplifier is turned off. The error amplifier works as a
hysteretic comparator. The PFM comparator senses the output voltage via the feedback pin and controls the
switching of the output FETs such that the output voltage ramps between ~0.8% and ~1.6% of the nominal PWM
output voltage (Figure 50). If the output voltage is below the ‘high’ PFM comparator threshold, the P1 & P2 (Buck
mode) or N2 & P1 (Boost mode) power switches are turned on. It remains on until the output voltage reaches the
‘high’ PFM threshold or the peak current exceeds the I
PFM
level set for PFM mode. The typical peak current in
PFM mode is: I
PFM
= 220 mA
Once the P1 (Buck mode) or N2 (Boost mode) power switch is turned off, the N1 & P2 (Buck mode) or P1 & P2
(Boost mode) power switches are turned on until the inductor current ramps to zero. When the zero inductor
current condition is detected, the N1(Buck mode) or P2 (Boost mode) power switches are turned off. If the output
voltage is below the ‘high’ PFM comparator threshold, the P1 & P2 (Buck mode) or N2 & P1 (Boost mode)
switches are again turned on and the cycle is repeated until the output reaches the desired level. Once the
output reaches the ‘high’ PFM threshold, the N1 & P2 (Buck mode) or P1 & P2 (Boost mode) switches are turned
on briefly to ramp the inductor current to zero, then both output switches are turned off and the part enters an
extremely low power mode. Quiescent supply current during this ‘sleep’ mode is 45µA (typ), which allows the
part to achieve high efficiency under extremely light load conditions.
Figure 50. PFM to PWM Mode Transition
In addition to the auto mode transition, the LM3668 operates in PFM Buck or PFM Boost based on the following
conditions. There is a small delta (~500 mV) known as dv1(~200 mV) & dv2(~300 mV) when V
OUT_TARGET
is very
close to V
IN
where the LM3668 can be in either Buck or Boost mode. For example, when V
OUT_TARGET
= 3.3V and
V
IN
is between 3.1V & 3.6V, the LM3668 can be in either mode depending on the V
IN
vs V
OUT_TARGET
.
• Region I: If V
IN
< V
OUT_TARGET
- dv1, the regulator operates in Boost mode.
• Region II: If V
OUT_TARGET
- dv1 < V
IN
< V
OUT_TARGET
+ dv2 ,the regulator operates in either Buck or Boost
mode.
• Region III: If V
IN
> V
OUT_TARGET
+ dv2, the regulator operates in Buck mode.
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