LM3668_08 Datasheet, PDF (13/18 Pages) National Semiconductor (TI) – 1A, High Efficiency Dual Mode Single Inductor Buck-Boost DC/DC Converter

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 con-
ditions. The internal current limit comparator will disable the
power device at a typical switch peak current limit of 1.85A
(typ.).
Under Voltage 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 re-
sumes when the temperature drops below 125°C.
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
2.5V. The start-up time thereby depends on the output ca-
pacitor and load current demanded at start-up. It is not rec-
ommended to start up the device at full load while in soft-start.
Application Information
SYNC/MODE PIN
If the SYNC/MODE pin is set high, the device is set to operate
at PWM mode only. If SYNC/MODE 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 SYNC/MODE pin can also be driven by an ex-
ternal clock to set the desired switching frequency between
1.6MHz to 2.7MHz.
V_SEL Pin
The LM3668 has built in logic for conveniently setting the out-
put 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 volt-
ages at 2.5V, the maximum operating current is 600mA and
800mA 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 op-
erating 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 ((Note 3) and (Note 5)), and maxi-
mum operating ambient of 85°C. As a result, the maximum
power dissipation using the above formula is around
1176mW. Refer to dissipation table below for P_D-MAX value at
different ambient temperatures.
Dissipation Rating Table
θ_JA
34°C/W ( 4
layers board
per JEDEC
standard)
T_A ≤ 25°C
2941mW
T_A ≤ 60°C
1912mW
T_A ≤ 85°C
1176mW
Inductor Selection
There are two main considerations when choosing an induc-
tor: 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 specifica-
tions are followed by different manufacturers so attention
must be given to details. Saturation current ratings are typi-
cally 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.
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