LM3668_15 Datasheet, PDF (19/30 Pages) Texas Instruments – 1-A, High-Efficiency Dual-Mode Single-Inductor Buck-Boost DC-DC Converter

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9 Application and Implementation
LM3668
SNVS449O – JUNE 2007 – REVISED APRIL 2015
NOTE
Information in the following applications sections is not part of the TI component
specification, and TI does not warrant its accuracy or completeness. TI’s customers are
responsible for determining suitability of components for their purposes. Customers should
validate and test their design implementation to confirm system functionality.
9.1 Application Information
9.1.1 MODE/SYNC Pin
If the MODE/SYNC pin is set high, the device is set to operate at PWM mode only. If MODE/SYNC pin is set low,
the device is set to automatically transition from PFM to PWM or PWM to PFM depending on the load current.
Do not leave this pin floating. The MODE/SYNC pin can also be driven by an external clock to set the desired
switching frequency between 1.6 MHz to 2.7 MHz.
9.1.2 VSEL Pin
The LM3668 has built in logic for conveniently setting the output voltage, for example if V_VSEL high, the output is
set to 3.3 V; with V_VSEL low the output is set to 2.8 V. It is not recommended to use this function for dynamically
switching between 2.8 V and 3.3 V or switching at maximum load.
9.2 Typical Application
VIN = 2.5V - 5.5V
C1
10 2F
2.2 2H
VDD
SW1
PVIN
VOUT
LM3668 ^FB
SW2 _SYNC/MODE
EN
VSEL
NC _{SGND PGND}
L = 2.8V
H = 3.3V
2.8V/3.3V
C2
22 2F
Figure 46. LM3668 Typical Application Circuit
9.2.1 Design Requirements
9.2.1.1 Maximum Current
The LM3668 is designed to operate up to 1 A. For input voltages at 2.5 V, the maximum operating current is 600
mA and 800 mA for 2.7 V input voltage. In any mode it is recommended to avoid starting up the device at
minimum input voltage and maximum load. Special attention must be taken to avoid operating near thermal
shutdown when operating in boost mode at maximum load (1 A). A simple calculation can be used to determine
the power dissipation at the operating condition; P_D-MAX = (T_J-MAX-OP – T_A-MAX)/R_θJA. The LM3668 has thermal
resistance R_θJA = 47.3°C/W (see Thermal Information and maximum operating ambient of 85°C. As a result, the
maximum power dissipation using the above formula is around 845 mW.
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