LTC3418 Datasheet, PDF (8/20 Pages) Linear Technology – 8A, 4MHz, Monolithic Synchronous Step-Down Regulator

LTC3418
U
OPERATIO
Dropout Operation
When the input supply voltage decreases toward the
output voltage, the duty cycle increases toward the maxi-
mum on-time. Further reduction of the supply voltage
forces the main switch to remain on for more than one
cycle eventually reaching 100% duty cycle. The output
voltage will then be determined by the input voltage minus
the voltage drop across the internal P-channel MOSFET
and the inductor.
Low Supply Operation
The LTC3418 is designed to operate down to an input
supply voltage of 2.25V. One important consideration at
low input supply voltages is that the R_DS(ON) of the
P-channel and N-channel power switches increases. The
user should calculate the power dissipation when the
LTC3418 is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in constant fre-
quency architectures by preventing subharmonic oscilla-
tions at duty cycles greater than 50%. It is accomplished
internally by adding a compensating ramp to the inductor
current signal. Normally, the maximum inductor peak
current is reduced when slope compensation is added. In
the LTC3418, however, slope compensation recovery is
implemented to keep the maximum inductor peak current
constant throughout the range of duty cycles. This keeps
the maximum output current relatively constant regard-
less of duty cycle.
Short-Circuit Protection
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. To
prevent current runaway from occurring, a secondary
current limit is imposed on the inductor current. If the
inductor valley current increases larger than 15A, the top
power MOSFET will be held off and switching cycles will be
skipped until the inductor current is reduced.
Voltage Tracking
Some microprocessors and DSP chips need two power
supplies with different voltage levels. These systems often
require voltage sequencing between the core power sup-
ply and the I/O power supply. Without proper sequencing,
latch-up failure or excessive current draw may occur that
could result in damage to the processor’s I/O ports or the
I/O ports of a supporting system device such as memory,
an FPGA or a data converter. To ensure that the I/O loads
are not driven until the core voltage is properly biased,
tracking of the core supply and the I/O supply voltage is
necessary.
Voltage tracking is enabled by applying a ramp voltage to
the TRACK pin. When the voltage on the TRACK pin is
below 0.8V, the feedback voltage will regulate to this
tracking voltage. When the tracking voltage exceeds 0.8V,
tracking is disabled and the feedback voltage will regulate
to the internal reference voltage.
Voltage Reference Output
The LTC3418 provides a 1.25V reference voltage that is
capable of sourcing up to 5mA of output current. This
reference voltage is generated from a linear regulator and
is intended for applications requiring a low noise reference
voltage. To ensure that the output is stable, the reference
voltage pin should be decoupled with a minimum of 2.2µF.
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