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dc2202af

DEMO MANUAL DC2202A

Description

LT8640

42V, 5A Micropower Synchronous

Step-Down Silent Switcher

Demonstration circuit 2202A is a 42V, 5A micropower

spectrum frequency modulation featuring the LT®8640.

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

light loads. Fast minimum on-time of 40ns enables high

TheLT8640switchingfrequencycanbeprogrammedeither

via oscillator resistor or external clock over a 200kHz to

3MHzrange.TheSYNCpinonthedemoboardisgrounded

(JP1 at BURST position) by default for low ripple Burst

Modeoperation.Tosynchronizetoanexternalclock,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

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.

performance summary

shows the LT8640 temperature rising on DC2202A demo

boardunderdifferentloadconditions.Theratedmaximum

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

SYMBOL

PARAMETER

CONDITIONS

MIN

TYP

MAX

UNITS

Input Supply Range

5.7

42

V

Output Voltage

4.85

5

5.15

V

Maximum Output Current

Thermal Conditions

5

A

Switching Frequency

1.85

2

2.15

MHz

EFE

Efficiency at DC

93.7

%