Datasheet

LM3668
SNVS449M –JUNE 2007–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
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.
VSEL Pin
The LM3668 has built in logic for conveniently setting the output voltage, for example if V
SEL
high, the output is
set to 3.3V; with V
SEL
low the output is set to 2.8V. It is not recommended to use this function for dynamically
switching between 2.8V and 3.3V or switching at maximum load.
Maximum Current
The LM3668 is designed to operate up to 1A. For input voltages at 2.5V, the maximum operating current is 600
mA and 800 mA for 2.7V 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 (1A). 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
)/θ
JA
. The LM3668 has thermal
resistance θ
JA
= 34°C/W
(1)(2)
and maximum operating ambient of 85°C. As a result, the maximum power
dissipation using the above formula is around 1176 mW. Refer to for P
D-MAX
value at different ambient
temperatures.
Dissipation Rating Table
θ
JA
T
A
≤ 25°C T
A
≤ 60°C T
A
≤ 85°C
34°C/W (4 layers board per
2941 mW 1912 mW 1176 mW
JEDEC standard)
(1) In applications where high power dissipation and/or poor package thermal resistance is present, the maximum ambient temperature may
have to be derated. Maximum ambient temperature (T
A-MAX
) is dependent on the maximum operating junction temperature (T
J-MAX-OP
=
125ºC), the maximum power dissipation of the device in the application (P
D-MAX
), and the junction-to ambient thermal resistance of the
part/package in the application (θ
JA
), as given by the following equation: T
A-MAX
= T
J-MAX-OP
– (θ
JA
× P
D-MAX
).
(2) All voltage is with respect to SGND.
Inductor Selection
There are two main considerations when choosing an inductor: the inductor should not saturate, and the inductor
current ripple should be small enough to achieve the desired output voltage ripple. Different saturation current
rating specifications are followed by different manufacturers so attention must be given to details. Saturation
current ratings are typically specified at 25°C. However, ratings at the maximum ambient temperature of
application should be requested from the manufacturer. Shielded inductors radiate less noise and should be
preferred.
In the case of the LM3668, there are two modes (Buck & Boost) of operation that must be consider when
selecting an inductor with appropriate saturation current. The saturation current should be greater than the sum
of the maximum load current and the worst case average to peak inductor current. The first equation shows the
buck mode operation for worst case conditions and the second equation for boost condition.
18 Submit Documentation Feedback Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LM3668