LM3668_15 Datasheet, PDF (15/30 Pages) Texas Instruments – 1-A, High-Efficiency Dual-Mode Single-Inductor Buck-Boost DC-DC Converter

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Feature Description (continued)
P1 SW1
VIN
+
-
SW2
P2
N2
LM3668
SNVS449O – JUNE 2007 – REVISED APRIL 2015
Load
Figure 42. Simplified Circuit for Boost Operation
8.3.3 Internal Synchronous Rectification
While in PWM mode, the LM3668 uses an internal MOSFET as a synchronous rectifier to reduce rectifier forward
voltage drop and associated power loss. Synchronous rectification provides a significant improvement in
efficiency whenever the output voltage is relatively low compare to the voltage drop across an ordinary rectifier
diode.
8.3.4 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 1.85 A (typ.).
8.3.5 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 2 V.
8.3.6 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.
8.3.7 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.
8.3.8 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.
8.3.9 Start-Up
The LM3668 has a soft-start circuit that smooth the output voltage and ramp current during start-up. During start-
up 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 2.5 V. The start-up time thereby
depends on the output capacitor and load current demanded at start-up. It is not recommended to start up the
device at full load while in soft-start.
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