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dc1973af

DEMO MANUAL DC1973A

Description

LT8697

USB 5V 2.5A Output, 42V Input Synchronous

Buck with Cable Drop Compensation

Demonstration circuit 1973A is a USB 5V 2.5A output, 42V

input synchronous buck with cable drop compensation

featuring the LT®8697. The LT8697 is a compact, high

efficiency,highspeedsynchronousmonolithicstep-down

switching regulator designed to power 5V USB applica-

tions. Top and bottom power switches, compensation

components and other necessary circuits are inside of

the LT8697 to minimize external components and simplify

design. A precise output voltage and programmable cable

drop compensation maintain accurate 5V regulation at the

USB socket at the end of a long cable.

The circuit runs at 2MHz to minimize external components

size and to avoid AM band. The demo board has an EMI

filter installed. The conducted EMI performance of the

board is shown on Figure 2. The figure shows the circuit

passes the EN55022 Class B with a wide margin.

When the load current is being drawn from VCABLE, the

load current is limited by the current limit of the LT8697

or by 5.8V limit at VOUT, whichever comes first. The

rated load current of the demonstration circuit is set at

2.1A, enough for all USB applications. If higher voltage or

higher current is needed at VCABLE, refer to application

examples on the data sheet. If VCABLE is not connected

to any load, VOUT can be used to power up a load. VSYS

can also be used to supply a small load.

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.

Whenprobingtheboard,payattentiontoGNDandC_GND.

Onthedemonstrationboard,theyareconnectedtogetherby

default through R13 so a user can connect the ground clip

of a probe to C_GND. However, if a user wants to evaluate

an actual cable, the cable will replace R4 and R13. The

copper between pads of R13 should be cut open. In this

case, C_GND is no longer the same as GND. A differential

measurement is needed to probe across the point of load.

The demonstration board also includes a USB socket.

With the proper configuration of jumpers X1, X2, X3 and

X4, the USB can be configured for different applications.

See Table 1 for configurations.

A 500mA onboard step load can also be activated to evalu-

ate the load transient response of the circuit. To active the

step load, simply set jumper X5 to V+.

The LT8697 data sheet gives a complete description of

the part, operation and application information. The data

sheet must be read in conjunction with this quick start

guide for demo circuit 1973A.

Design files for this circuit board are available at

http://www.linear.com/demo/DC1973A