Datasheet

V
OUT (Target)
Buck
or
Boost
Boost Buck
Region I
Region II Region III
V
IN
dV1 - V
OUT (TARGET)
V
OUT (TARGET)
+ dV2
LM3668
www.ti.com SNVS449M –JUNE 2007–REVISED MAY 2013
Figure 51. V
OUT
vs V
IN
Transition
In the buck PFM operation, P2 is always turned on and N2 is always turned off , P1 and N1 power switches are
switching. P1 and N1 are turned off to enter " sleep mode" when the output voltage reaches the "high"
comparator threshold. In boost PFM operation, P2 and N2 are switching. P1 is turned on and N1 is turned off
when the output voltage is below the "high" threshold. Unlike in buck mode, all four power switches are turned off
to enter "sleep" mode when the output voltage reaches the "high" threshold in boost mode. In addition, the
internal current sensing of the I
PFM
is used to determine the precise condition to switch over to buck or boost
mode via the PFM generator.
Current Limit Protection
The LM3668 has current limit protection to prevent excessive stress on itself and external components during
overload conditions. The internal current limit comparator will disable the power device at a typical switch peak
current limit of 1.85A(typ.).
Undervoltage Protection
The LM3668 has an UVP comparator to turn the power device off in case the input voltage or battery voltage is
too low . The typical UVP threshold is around 2V.
Short Circuit Protection
When the output of the LM3668 is shorted to GND, the current limit is reduced to about half of the typical current
limit value until the short is removed.
Shutdown
When the EN pin is pulled low, P1 and P2 are off; N1 and N2 are turned on to pull SW1 and SW2 to ground.
Thermal shutdown
The LM3668 has an internal thermal shutdown function to protect the die from excessive temperatures. The
thermal shutdown trip point is typically 150°C; normal operation resumes when the temperature drops below
125°C.
Startup
The LM3668 has a soft-start circuit that smooth the output voltage and ramp current during startup. During
startup the bandgap reference is slowly ramped up and switch current limit is reduced to half the typical value.
Soft start is activated only if EN goes from logic low to logic high after V
IN
reaches 2.5V. The startup time thereby
depends on the output capacitor and load current demanded at startup. It is not recommended to start up the
device at full load while in soft-start.
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