LTC3618 Datasheet, PDF (11/24 Pages) Linear Integrated Systems – Dual 4MHz, 3A Synchronous Buck Converter for DDR Termination

LTC3618
OPERATION
Pulse-Skipping Mode Operation
Connecting the MODE/SYNC pin to SV_IN enables pulse-
skipping mode for V_DDQ only. As the load current decreases,
the peak inductor current will be determined by the voltage
on the ITH1 pin until the ITH1 voltage drops below 550mV,
corresponding to 0A. At this point switching cycles will be
skipped to keep the output voltage in regulation.
Forced Continuous Mode Operation
In forced continuous mode the inductor current is con-
stantly cycled which creates a minimum output voltage
ripple at all output current levels.
Connecting the MODE/SYNC pin to ground will select the
forced continuous mode operation for V_DDQ.
The forced continuous mode must be used if the output
is required to sink current.
Dropout Operation
As the input supply voltage approaches the output voltage,
the duty cycle increases toward the maximum 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 LTC3618 is designed to operate down to an input supply
voltage of 2.25V. An important consideration at low input
supply voltages is that the R_DS(ON) of the P-channel and
N-channel power switches increases by 50% compared to
5V. The user should calculate the power dissipation when
the LTC3618 is used at 100% duty cycle with low input
voltages to ensure that thermal limits are not exceeded.
LTC3618 implements slope compensation by adding a
compensation ramp to the inductor current signal.
Short-Circuit Protection
The peak inductor current at which the current comparator
shuts off the top power switch is controlled by the voltage
on the ITH pin.
If the output current increases, the error amplifier raises
the ITH pin voltage until the average inductor current
matches the new load current. In normal operation, the
LTC3618 clamps the maximum ITH pin voltage at ap-
proximately 1.05V which corresponds to about 5.5A peak
inductor current.
When the output is shorted to ground, the inductor current
decays very slowly during a single switching cycle. The
LTC3618 uses two techniques to prevent current runaway
from occurring:
1. If the output voltage drops below 50% of its nominal
value, the clamp voltage at the ITH pin is lowered,
causing the maximum peak inductor current to lower
gradually with the output voltage. When the output volt-
age reaches 0V, the clamp voltage at the ITH pin drops
to 40% of the clamp voltage during normal operation.
The short-circuit peak inductor current is determined by
the minimum on-time of the LTC3618, the input voltage
and the inductor value. This foldback behavior helps
in limiting the peak inductor current when the output
is shorted to ground. It is disabled during internal or
external soft-start and tracking up/down operation (see
the Applications Information section).
2. If the inductor current of the bottom MOSFET increases
beyond 6A typical, the top power MOSFET will be held
off and switching cycles will be skipped until the induc-
tor current reduces.
Slope Compensation and Inductor Peak Current
Slope compensation provides stability in current mode
constant-frequency architectures by preventing subhar-
monic oscillations at duty cycles greater than 50%. The
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