1
dc2468af
DEMO MANUAL DC2468A
DESCRIPTION
LT8645S
65V, 8A Micropower Synchronous
Step-Down Silent Switcher 2
Demonstration circuit 2468A is a 65V, 8A micropower syn-
chronous step-down second generation Silent Switcher
®
with spread spectrum frequency modulation featuring the
. The demo board is designed for 5V output from
a 5.6V to 65V input. The wide input range allows a variety
of input sources, such as automotive batteries and indus-
trial supplies. The LT8645S is a compact, ultralow emis-
sion, high efficiency, and high speed synchronous mono-
lithic 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. Ultralow quiescent cur-
rent in Burst Mode
®
operation achieves high efficiency at
very light loads. Fast minimum on-time of 40ns enables
high V
IN
to low V
OUT
conversion at high frequency.
The LT8645S switching frequency can be programmed
either via oscillator resistor or external clock over a
200kHz to 2.2MHz range. The default frequency of demo
circuit 2468A 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 pulse
skipping 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
from VIN terminal to bypass the EMI filter). Figure 2 shows
the LT8645S temperature rising on DC2468A demo board
under different load conditions. The rated maximum load
L
, LT, LTC, LTM, Linear Technology, the Linear logo, Silent Switcher and Burst Mode are
registered trademarks of Analog Devices, Inc. All other trademarks are the property of their
respective owners.
current is 8A, while derating is necessary for certain input
voltage and thermal conditions. Low switching frequency
can extend the output load capability by reducing the
power dissipations. Figure 3 shows the temperature ris-
ing at 500kHz switching frequency.
The demo board has an EMI filter installed. The EMI
performance of the board (with EMI filter) is shown on
Figure 4. 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 4, the input EMI
filter is required and the input voltage should be applied
at VEMI 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
VIN terminal.
The LT8645S 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 demo circuit 2468A. The LT8645S is assembled in a
6mm × 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.
Design files for this circuit board are available at
http://www.linear.com/demo/DC2468A
PERFORMANCE SUMMARY
Specifications are at T
A
= 25°C
SYMBOL
PARAMETER
CONDITIONS
MIN
TYP
MAX
UNITS
V
IN_EMI
Input Supply Range with EMI Filter
5.6
65
V
V
OUT
Output Voltage
4.85
5
5.15
V
I
OUT
Maximum Output Current
Derating is Necessary for Certain V
IN
and Thermal Conditions
8
A
f
SW
Switching Frequency
1.85
2
2.15
MHz
EFF
Efficiency
V
IN
= 12V, I
OUT
= 4A
93.8
%
