LM3668_15 Datasheet, PDF (17/30 Pages) Texas Instruments – 1-A, High-Efficiency Dual-Mode Single-Inductor Buck-Boost DC-DC Converter

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Device Functional Modes (continued)
LM3668
SNVS449O – JUNE 2007 – REVISED APRIL 2015
PFM Mode at Light Load
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
Low2 PFM Threshold,
switch back to PWMmode
Load current
increases
High PFM Threshold
~1.016*Vout
Low1 PFM Threshold
~1.008*Vout
Current load
increases,
draws Vout
towards
Low2 PFM
Threshold
Low2 PFM Threshold
Vout
PWM Mode at
Moderate to Heavy
Loads
Figure 44. 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 (approximately 500 mV) known as dv1 (approximately 200 mV) and dv2
(approximately 300 mV) when V_{OUT_TARGET} is very close to V_IN where the device can be in either Buck or Boost
mode. For example, when V_{OUT_TARGET} = 3.3 V and V_IN is between 3.1 V and 3.6 V, 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.
Region I
Region II
Region III
V_{OUT (Target)}
Boost
Buck
or
Boost
Buck
dV1 - V_{OUT (TARGET)}
V_{OUT (TARGET)} + dV2
V_IN
Figure 45. V_OUT vs V_IN Transition
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