Linear Technology LT8640 Manual

Description
Demonstration circuit 2202A is a 42V, 5A micropower
synchronous step-down Silent Switcher
spectrum frequency modulation featuring the
The demo board is designed for 5V output from a 5.7V
to 42V input. The wide input range allows a variety of
input sources, such as automotive batteries and industrial
supplies. The LT8640 is a compact, ultralow emission,
high efficiency, and high speed synchronous monolithic
step-down switching regulator. The integrated power
switches and inclusion of all necessary circuitry reduce
the components count and solution size. Special Silent
Switcher architecture minimizes EMI/EMC emissions.
Selectable spread spectrum mode can further improve
EMI/EMC performance. Ultralow 2.5μA quiescent current
operation achieves high efficiency at very
in Burst Mode
®
light loads. Fast minimum on-time of 40ns enables high
V to low V conversion at high frequency.
IN OUT
The LT8640 switching frequency can be programmed either
via oscillator resistor or external clock over a 200kHz to
3MHz range. 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 tur-
ret. 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 and 24V input in
Burst Mode operation (input from V
performance summary
SYMBOL PARAMETER
V Input Supply Range
IN
V Output Voltage
OUT
I Maximum Output Current
OUT
f Switching Frequency
SW
EFE Efficiency at DC
42V, 5A Micropower Synchronous
with spread
®
LT 8640.
®
turret pin). Figure 2
IN
Specifications are at T
CONDITIONS
Derating Is Necessary for Certain V
Thermal Conditions
V = 12V , I
IN OUT
DEMO MANUAL DC2202A
Step-Down Silent Switcher
shows the LT8640 temperature rising on DC2202A demo
board under different load conditions. The rated maximum
load current is 5A, while derating is necessary for certain
input voltage and thermal conditions.
The demo board has an EMI filter installed. The EMI
performance of the board (with EMI filter) is shown on
Figure 3. The red line in Figure 3 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 VEMI turret pin. An
inductor L2, which is a 0Ω jumper on the board by default
now, can be added in the EMI filter to further reduce the
conducted emission.
The LT8640 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 2202A. The LT8640 is assembled in a
3mm × 4mm plastic QFN package with exposed pads for
low thermal resistance. Proper board layout is essential
for both low EMI operation and maximum thermal perfor-
mance. See the data sheet sections 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/DC2202A
L, LT, LTC, LTM, Linear Technology, the Linear logo, Burst Mode and Silent Switcher are
registered trademarks of Linear Technology Corporation. All other trademarks are the property
of their respective owners.
= 25°C
A
and
IN
= 3A
LT8640
MIN TYP MAX UNITS
5.7 42
4.85 5 5.15
5
1.85 2 2.15
93.7
dc2202af
V
V
A
MHz
%
1