LTC3815_15 Datasheet, PDF (16/42 Pages) Linear Technology – 6A Monolithic Synchronous DC/DC Step-Down Converter with Digital Power System Management

LTC3815
Operation
25MHz Error Amplifier and Remote Sense Differential
Amplifier
The LTC3815 utilizes a 25MHz error amplifier and dif-
ferential amplifier for fast and accurate output voltage
regulation. The operational amplifier style error amplifier
allows precision tuning of the system poles and zeros for
optimal transient response. The remote sense differential
amplifier allows output voltage sensing at the point-of-load
and thus provides very accurate regulation of the output
voltage and telemetry readback regardless of load current.
The sensed output voltage is available at the DA_OUT pin
(referenced to SGND). This pin is typically connected to
the FB pin which is the error amplifier “-“ input.
Using Separate VIN/PVIN Supplies
The LTC3815 has two supply pins: V_IN that supplies the IC
control circuitry and PV_IN that supplies the driver and power
switches. V_IN requires a minimum of 2.25V to guarantee
proper operation, while PV_IN may be able to supply power
to the load at lower voltages if the load demands do not
exceed the weaker capability of the power switches at the
lower voltage. To maximize the lower operating range of
the supply voltage, two separate supplies can be used – a
V_IN supply that is > 2.25V and rated at 10mA or higher
and a PV_IN supply, rated for the load, that can be run all
the way into dropout. Each supply pin has an individual
undervoltage-lockout comparator to shut off the supply
when its respective voltage is too low to guarantee proper
operation.
Thermal Warning and Thermal Shutdown
The LTC3815 has two levels of thermal thresholds
and two levels of responses. When the internal die
temperature exceeds 150°C, the overtemperature bit in
the STATUS_WORD is set and the ALERT pin pulls low
to alert the PMBus master. If the temperature continues
to rise and exceeds 170°C, the LTC3815 shuts down all
circuitry, including output regulation, and will no longer
respond to the PMBus host. Both temperature monitors
have about 20°C of hysteresis before the overtemperature
condition is cleared. The temperature warning bit in the
STATUS_WORD is latched and remains set until the host
clears it.
2-Phase Operation
For output loads that demand more than 6A of current, two
LTC3815’s can be paralleled to run out-of-phase to provide
up to 12A output current. To configure a 2-phase system,
one LTC3815 will act as a master and the other a slave
(see the schematic in the Typical Applications section).
Connecting the V_{SS_SENSE} pin to V_IN puts the LTC3815 in
slave mode by tri-stating its error amplifier and remote
sense amplifier. The I_TH pins of both IC’s are connected
together so that both are regulating the inductor current
based on the master’s I_TH voltage. The master’s CLKOUT
pin is a clock waveform that is 180° out-of-phase to its
internal clock. This CLKOUT can be connected to the
MODE/SYNC pin of the slave to force the slave’s PLL to
lock onto this clock input and run out-of-phase with the
master. The RUN_STBY pins are also connected together
as a handshaking signal between the two so that both will
shutoff together in case of a fault in only one of the phases,
such as overtemperature condition.
See the Applications Information section for further details
regarding 2-phase operation.
Discontinuous/Forced Continuous Operation
The LTC3815 can operate in one of two modes selectable
with the MODE/SYNC pin: discontinuous mode or forced
continuous mode. Connecting the MODE/SYNC pin to
V_IN selects discontinuous mode. Discontinuous mode is
selected when high efficiency at very light loads is desired.
In this mode, when the inductor current reverses, the bot-
tom MOSFET turns off to minimize the efficiency loss due
to reverse current flow. This reduces the conduction loss
and slightly improves the efficiency. As the load reduces,
the driver switching frequency drops in proportion to the
load, which further improves efficiency by minimizing
gate charge losses.
Forcing the MODE/SYNC pin low enables forced continuous
mode operation. In forced continuous mode, the bottom
MOSFET is always on when the top MOSFET is off, allowing
the inductor current to reverse at low currents. This mode
is less efficient due to conduction and switching losses,
but has the advantage of better transient response at low
currents, constant frequency operation, and the ability to
maintain regulation when sinking current.
3815p
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For more information www.linear.com/LTC3815