LTM2883 datasheet(16/24 Pages) AD

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LTM2884

Rev. D

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APPLICATIONS INFORMATION

These supplies are available to support interface logic to

the isolated USB port. In order to meet the suspend mode

current limit, minimize the DC current of external appli-

cations on the VLO output supply. VLO and VLO2 are pro-

tected from overcurrent and overtemperature conditions.

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

transceiver and to the VLO pin. The VCC input supplies

power to VCC2 and VLO2 through an isolated DC/DC con-

verter. 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 opera-

tion. 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 con-

verter remains on during suspend, therefore power and

enumeration information is retained. The VCC supply con-

sumes 50mA to support the isolated power during sus-

pend. 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 down-

stream side of the LTM2884 and VBUS and VCC are con-

nected together, the input current is higher due to the

operating current and the efficiency of the DC/DC con-

verter. 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

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