Analog Devices DC2779A Demo Manual

DESCRIPTION
Demonstration circuit 2779A is an 8V, 16A synchronous
step-down Silent Switcher
quency modulation featuring the
board is designed for 1.2V output from a 2.7V to 8V input.
The LT8644S is a compact, ultralow emission, high effi-
ciency, and high speed synchronous monolithic step-
down switching regulator. The integrated bypass capaci-
tors optimize all the fast current loops and make it easier
to minimize EMI emissions by reducing layout sensitivity.
Selectable spread spectrum mode further improves EMI
performance, making it perfect solution to the noise sen-
sitive applications.
Peak current mode control with minimum on-time of as
low as 20ns allows high step-down conversion even at
high frequency. The LT8644S switching frequency can be
programmed either via an external resistor or an external
clock over a 300kHz to 3.5MHz range. The default fre-
quency of demo circuit 2779A is 2MHz.
The SYNC/MODE pin on the demo board DC2779A is
grounded (JP1 at BURST position) by default for low
ripple Burst Mode
different operation modes through JP1 and SYNC termi-
nal (Table 1).
Table 1. Operation Mode Configuration
JP1 Position
SPREAD-SPECTRUM
BURST
FCM/SYNC
FCM/SYNC
Arrow.com.
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2 with spread spectrum fre-
®
LT 8644S. The demo
®
operation. It can be configured into
®
SYNC (E5) Input Operation Mode
N/A Forced Continuous Mode with
Spread-Spectrum
N/A Low Ripple Burst Mode
Floating Forced Continuous Mode
without Spread-Spectrum
External Clock Forced Continuous Mode with
Synchronization
DEMO MANUAL DC2779A
8V, 16A Synchronous Step-Down
Figure 4 shows the efficiency of the circuit at 3.3V input
and 5V input in force continuous mode operation (input
from VIN terminal). To get accurate efficiency measure-
ment, measure the input voltage at the VIN_SENSE/GND
terminals and measure the output voltage at the VOUT+
SENSE/VOUT– SENSE terminals. Figure 5 shows the
LT8644S temperature rise on DC2779A demo board at
3.3V input and 5V input in forced continuous mode.
The LT8644S is assembled in a 4mm × 4mm LQFN pack-
age with exposed pads for low thermal resistance. The
rated maximum continuous load current is 16A, while
derating may be necessary for certain input voltage and
thermal conditions.
The demo board has an EMI filter installed. This EMI
filter can be included by applying the input voltage at
the VEMI terminal. The EMI performance of the board
(with EMI filter) is shown on Figure 6. The red lines in
EMI Performance are CISPR25 Class 5 limits. The figure
shows that the LT8644S circuit passes the test with a
wide margin. To achieve EMI performance as shown in
Figure 6, the input EMI filter is required and the input
voltage should be applied to VEMI terminal, and the test
setup can be referred to the CISPR25 standards. If the
input is applied to VIN terminal, the EMI filter is bypassed.
The LT8644S data sheet gives a complete description of
the part, the operation and application information. The
data sheet must be read in conjunction with this demo
manual for demo circuit 2779A. The layout recommen-
dations for low EMI operation and maximum thermal
performance are available in the data sheet section Low
EMI PCB Layout and Thermal Considerations. Contact ADI
applications engineer for support.
Design files for this circuit board are
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LT8644S
Silent Switcher 2
available.
Rev. 0
1