DEMO MANUAL DC1789A
LTM2884
Isolated USB Transceiver
with Isolated Power
Description
Demonstration circuit 1789A is an isolated USB transceiver
with isolated power featuring the
LTM
®
2884.
The demo
circuit features an EMI optimized circuit configuration and
printed circuit board layout. All components are integrated
into the µModule
®
isolator. The demo circuit operates
from a supply on V
CC
and/or V
BUS
. The part generates
an isolated output voltage on V
CC2
and communicates all
necessary signaling across the isolation barrier through
LTC’s isolator µModule technology.
Design files for this circuit board are available at
http://www.linear.com/demo
L,
LT, LTC, LTM, Linear Technology, the Linear logo and µModule are registered trademarks of
Linear Technology Corporation. All other trademarks are the property of their respective owners.
performance summary
SYMBOL
V
CC
V
BUS
V
CC2
t
LDR
t
FDR
V
IORM
PARAMETER
(T
A
= 25°C)
CONDITIONS
MIN
4.4
4.4
V
CC
= V
BUS
= 4.4V, I
CC2
= 200mA
V
CC
= 8.1V, I
CC2
= 500mA
4.75
4.75
TYP
12
5
5
5
1.5
12
GND to GND2
560
400
30
MAX
16.5
16.5
5.25
5.25
UNITS
V
V
V
V
Mbps
Mbps
V
RMS
kV/µs
V
DC
Operating Supply Range (Isolated Power Input)
Operating Supply Range (USB Bus Power Input)
Regulated Output Voltage
Low Speed Data Rate
Full Speed Data Rate
Maximum Working Insulation Voltage
Common Mode Transient Immunity
operating principles
The LTM2884 contains an isolated DC/DC converter de-
livering power to V
CC2
at 5V from the input supply V
CC
and/or V
BUS
. Isolation is maintained by the separation of
GND and GND2 where significant operating voltages and
transients can exist without affecting the operation of the
LTM2884. The logic side ON pin enables or shuts down
the LTM2884. All logic side signals are referenced to the
logic supply pin V
LO
. The LTM2884 has two power supply
inputs, V
CC
and V
BUS
. For applications requiring more
than 200mA from V
CC2
, V
CC
must be connected to an
external supply of 8.1V to 16.5V. V
BUS
may be connected
to USB bus power or to the external supply. For applica-
tions requiring 200mA or less connect V
CC
and V
BUS
to
USB bus power.
Upstream USB signaling is controlled by the bidirectional
pins D1
+
and D1
–
. A 1.5k pull-up resistor is automatically
configured dependent upon the connected downstream
peripheral device. For full speed and low speed devices
the pull-up is asserted on D1
+
and D1
–
, respectively. The
downstream USB data pins, D2
+
and D2
–
, each have
integrated 15k pull-down resistors.
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DEMO MANUAL DC1789A
operating principles
The LTM2884 includes a suspend power feature that
when enabled (high) reduces the supply current on V
CC
and V
BUS
when the upstream USB bus is idle. However, in
this mode, if a downstream device is connected or discon-
nected from the bus or remote wake-up functionality is
configured, it will not be recognized by the LTM2884 and
relayed to the host. A resume command at the upstream
side will wake up the LTM2884 and a re-numeration by
the host will be required. If suspend power is configured
low the LTM2884 operates in a low power mode but will
respond to disconnects, reconnects, downstream wake-up
commands, or host resume commands.
The demo circuit has been designed and optimized for
low RF emissions. EMI mitigation techniques used include
the following:
1. Board/ground plane size has been minimized. This
reduces the dipole antenna formed between the logic
side and isolated side ground planes.
2. Top signal routing and ground floods have been opti-
mized to reduce signal loops, minimizing differential
mode radiation.
EMI performance is shown in Figure 1, measured using
a gigahertz transverse electromagnetic (GTEM) cell and
method detailed in IEC 61000-4-20, “Testing and Mea-
surement Techniques – Emission and Immunity Testing
in Transverse Electromagnetic Waveguides.”
54
48
42
36
30
dBµV/m
24
18
12
6
0
–6
–12
–18
0
DETECTOR = PEAK-HOLD
R
BW
= 120kHz, V
BW
= 300kHz
SWEEP TIME = 680ms, ≥10 SWEEPS
# OF POINTS = 501
100 200 300 400 500 600 700 800 900 1000
FREQUENCY (MHz)
DC1789A F01
CISPR 22 CLASS B LIMIT
DC1789A
Figure 1. DC1789A Radiated Emissions,
Normalized to 10m per IEC 61000-4-20
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DEMO MANUAL DC1789A
Quick start proceDure
Demonstration circuit 1789A makes it easy to set up and
evaluate the performance of the LTM2884. Refer to Figure
2 for proper measurement equipment setup and follow
the procedure below.
NOTE: When measuring the input or output voltage ripple
or high speed signals, care must be taken to avoid a long
ground lead on the oscilloscope probe.
1. Place jumpers in their default positions:
JP1
V
CC
supply in the V
BUS
position
JP2
V
BUS
supply in the V
BUS
position
JP3
SUSPEND POWER in the OFF position
JP4
C2 in the OUT position
NOTE: To simulate a hub/splitter output, C2 can be
jumpered IN. The additional capacitance reduces V
BUS
droop on downstream device plug-in.
2. Connect USB cable from computer to input side (J1)
of the demo board.
3. Connect computer mouse, low speed device, or USB
memory stick, typically high speed device, to output
side (J2) of demo board.
4. Verify proper operation of mouse or memory stick.
MEMORY STICK
MOUSE
Figure 2. Demo Board Setup
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DEMO MANUAL DC1789A
pcB layout
Layer 1. Top Layer
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DEMO MANUAL DC1789A
pcB layout
Layer 2. Bottom Layer
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