Analog Devices DC2851A User Manual

DC2851A User Guide
General Description
Evaluation kit DC2851A is a quad-channel synchronous
step-down regulator featuring the LT®7200S. The
LT7200S is ideal for high step-down ratio applications
that operate at high frequency while demanding fast
transient response thanks to its constant frequency,
current-mode architecture, and phase-lockable controlled
on-time. The DC2851A operates from an input voltage
range of 2.9V to 18V, with each regulator capable of
delivering ±5A of output current. The evaluation kit is
designed for 1.2V, 1.8V, 3.3V, and 5V outputs with
switching frequency set at 1MHz. The DC2851A provides
LT7200S-based different multichannel combinations for
flexible output current rating.
The LT7200S features independent soft-start, output
tracking and power good for each output to simplify the
complex design of quad-output converters. The output
tracking and soft-start pins allows to control the rise time
and ramp rate of the output voltage. Each output can be
independently disabled into the low quiescent current
shutdown mode with its own TRACK or RUN pin.
A user-selectable MODE/SYNC pin on the evaluation kit
allows the part to operate in either discontinuous mode
for higher efficiency at light load currents, or forced
continuous mode for lower output ripple at the expense of
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Quad 18V, ±5A Synchronous
Monolithic Step-Down Regulator
light load efficiency. The MODE/SYNC pin can also be
used to synchronize the switching frequency to an
external clock. The switching frequency for all regulators
can be programmed either through the oscillator resistor
or external clock over a 400kHz to 3MHz range. At all
frequencies, a 180° phase shift is maintained between
channel 1 and channel 2, channel 3 and channel 4 when
PHMODE pin is low, reducing the input peak current and
voltage ripple.
The DC2851A has an EMI filter installed on the bottom
layer. The conducted and radiated EMI performance of
the board is shown in Figure 4. The red line in Figure 4 is
the CISPR25 Class 5 peak limit. The figure shows that
the circuit passes the test with a wide margin. To achieve
EMI/ EMC performance as shown in Figure 4, the input
EMI filter is required, and the input voltage should be
applied at VEMI turret.
The LT7200S data sheet gives a complete description of
the part operation and application information. Read the
data sheet in conjunction with this quick start guide for
DC2851A. The LT7200S is assembled in a 5mm × 6mm
LQFN package with exposed pads for low thermal
resistance. Proper board layout is essential for both low
EMI operation and best thermal performance.
LT7200S
Rev 0; 10/22