LTM2884
Applications Information
Supply Current
Loading the multiple output supply pins of the LTM2884
affects the supply current on VBUS and VCC. The VBUS input
supplies current to the the upstream side of the trans-
ceiver and to the VLO pin. The VCC input supplies power
to VCC2 and VLO2 through an isolated DC/DC converter.
The efficiency (η) of the DC/DC converter is shown in the
Typical Performance Characteristics section for 5V and
12V inputs from VCC to VCC2.
Supply Current Equations
Operating:
IBUS = 6mA + IVLO
( ) ICC = VCC2 •
6mA + ICC2 + IVLO2
η • VCC
Suspend: SPNDPWR = 0
IBUS = 1.5mA + IVLO
( ) ICC = VCC2 •
6mA + ICC2 + IVLO2
η • VCC
Suspend: SPNDPWR = VLO
IBUS = 0.45mA + IVLO
ICC
=
VCC
45k
Off:
IBUS = 10µA
ICC
=
VCC
45k
USB 2.0 Compatibility
The LTM2884 µModule transceiver is compatible with the
USB 2.0 specification of full and low speed operation. Some
characteristics and implementations may not support full
compliance with the USB 2.0 specification. Three specific
cases exist within the LTM2884 µModule transceiver and
the integrated DC/DC power converter.
First, the propagation delay for full speed data of 80ns
exceeds the specification for a single hub of 44ns plus
the attached cable delay of 26ns. This is due to driving
the signal to the 3.3V rail prior to a K-state transition to
maintain balanced crossover voltages equivalent to the
cross over voltages of the successive data transitions.
USB ports commonly drive the idle state bus to the 3.3V
rail prior to the k-state start of packet transition.
Second, setting SPNDPWR = VLO will cause the DC/DC
power converter to turn off during a bus suspend. VCC2
will lose power causing the downstream device to lose
enumeration. Remote wake-up, disconnect, and recon-
nect events are ignored. A resume command from the
host or upstream hub will start the DC/DC converter and
wake up the downstream device. The downstream device
will require re-enumeration, which causes a failure in USB
compliance testing. After a resume command initiates,
a delay of 3ms will elapse before the isolated device is
fully powered. When SPNDPWR = 0V, the DC/DC power
converter remains on during suspend, therefore power
and enumeration information is retained. The VCC supply
consumes 50mA to support the isolated power during
suspend. Separate the VBUS and VCC supplies to comply
with the 2.5mA USB 2.0 VBUS suspend current specification.
Third, when connecting a low power device to the
downstream side of the LTM2884 and VBUS and VCC
are connected together, the input current is higher due
to the operating current and the efficiency of the DC/DC
converter. The operating current of the DC/DC converter
and the USB transceiver function is 46mA. The efficiency
of the converter is approximately 55%, resulting in a 1/0.55
increase in the input current due to the load current on
VCC2. A 100mA load on VCC2 appears as a 181mA load
+ operating current at VBUS and VCC. In order to meet a
100mA input current, the VCC2 load current must be less
than 25mA. This characteristic of an isolated supply may
limit the use of the LTM2884 in bus powered hub applica-
tions or downstream connection to a bus powered hub.
Connect VCC to an external supply to mitigate this concern.
2884f
16
For more information www.linear.com/LTM2884