Analog Devices Linear Technology 2658A Demo Manual

Description
Demonstration circuit 2658A is a 42V, 6A synchronous
step-down second generation SILENT SWITCHER with
spread spectrum frequency modulation featuring the
LT 8643S. The demo board is designed for 5V output
®
from a 5.8V to 42V input. The wide input range allows a
variety of input sources, such as automotive batteries and
industrial supplies. The LT8643S is a compact, ultralow
emission, high efficiency, and high speed synchronous
monolithic step-down switching regulator. The integrated
bypass capacitors optimize all the fast current loops and
make it easier to minimize EMI/EMC emissions by reducing
layout sensitivity. Selectable spread spectrum mode can
further improve EMI/EMC performance. Fast minimum
on-time of 30ns enables high V
at high frequency.
The LT8643S switching frequency can be programmed
either via oscillator resistor or external clock over a
200kHz to 3MHz range. The default frequency of demo
circuit 2658A is 2MHz. The SYNC pin on the demo board
is grounded (JP1 at BURST position) by default for low
ripple burst mode operation. To synchronize to an external
clock, move JP1 to SYNC and apply the external clock to
the SYNC terminal. Spread spectrum mode and forced
continuous mode can be selected respectively by moving
JP1 shunt. Figure 1 shows the efficiency of the circuit at
12V input and 24V input in Burst Mode Operation (input
terminal to bypass the EMI filter). Figure 2 shows
from V
IN
performance summary
SYMBOL PARAMETER
V Input Supply Range with EMI Filter
IN_EMI
V Output Voltage
OUT
I Maximum Output Current
OUT
f Switching Frequency
SW
EFF Efficiency
Arrow.com.
Downloaded from
to low V conversion
IN OUT
Specifications are at T
CONDITIONS
Derating Is Necessary for Certain V
Thermal Conditions
V = 12V , I
IN OUT
DEMO MANUAL DC2658A
42V, 6A Synchronous
Step-Down Silent Switcher 2
the LT8643S temperature rising on DC2658A demo board
under different load conditions. The rated maximum load
current is 6A, while derating is necessary for certain input
voltage and thermal conditions.
The demo board has an EMI filter installed. The EMI perfor-
mance of the board (with EMI filter) is shown on Figure 3.
The red line in Radiated EMI Performance is 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 3, the input EMI filter is required and
the input voltage should be applied at VIN_EMI terminal.
An inductor can be added in the EMI filter to further reduce
the conducted emission. The EMI filter can be bypassed
by applying the input voltage at V
The LT8643S data sheet gives a complete description
of the part, operation and application information. The
data sheet must be read in conjunction with this demo
manual for DC2658A. The LT8643S is assembled in a
4mm × 4mm LQFN package with exposed pads for low
thermal resistance. The layout recommendations for low
EMI operation and maximum thermal performance are
available in the data sheet section Low EMI PCB Layout
and Thermal Considerations and Peak Output Current.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2658A
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
= 25°C
A
and
IN
= 3A
LT8643S
terminal.
IN
MIN TYP MAX
5.8 42
4.85 5 5.15
6
1.85 2 2.15
94.2
UNITS
V
V
A
MHz
%
dc2658af
1