LTM2884 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTM2884' PDF : 24 Pages View PDF
LTM2884
Applications Information
Suspend Mode
When the upstream USB bus is idle for greater than 3ms,
the LTM2884 enters suspend mode. The power savings
and behavior in suspend mode depend on the state of the
SPNDPWR pin, as summarized in Table 1.
Table 1. Suspend Mode Operation
SPNDPWR VCC2 IBUS
ICC
High
Off < 500µA VCC/45k
Low
On 1.5mA 50mA
WAKE-UP
WAKE-UP
TIME
Resume
3ms
Resume or
10µs
Remote Wake-Up
The biggest power savings in suspend mode comes when
SPNDPWR is high. In this case, the DC/DC converter is
disabled, shutting down power to the isolated side, while
the current draw on VCC and VBUS are minimized. How-
ever, in this mode, if a downstream device is connected or
disconnected from the bus or remote wake-up functionality
is configured, it will not be recognized by the LTM2884
and will not be 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. Recovery time
is about 3ms from the start of the resume command on
the upstream side.
If SPNDPWR is low in suspend mode, the LTM2884 oper-
ates in a low power mode but maintains a higher functional
state with the DC/DC converter on and the downstream
transceiver powered. The VBUS current is reduced to
1.5mA and VCC current is about 50mA when there is no
external draw on VCC2. Wake-up is initiated with discon-
nects, reconnects, or a remote wake-up command from a
downstream device or a resume command from the host.
Recovery time from suspend mode is about 10µs from
when the first state change is detected.
During suspend mode DC current drawn from VLO into
external circuits will be supplied from VBUS and may exceed
the limits set in the USB specification.
DC/DC Power Supply
The internal DC/DC converter converts the input power
from the VCC pin to the VCC2 output. The power delivered
to the VCC2 pin is regulated and current limited to protect
against overcurrent conditions. The voltage supply, VCC,
is sensed to limit the maximum current that can be delivered
before USB specifications are exceeded. Connecting the VCC
and VBUS supply pins to the USB VBUS pin (4.4V to 5.5V) lim-
its the maximum downstream side supply current to 200mA
before VCC2 supply degradation. When VCC is connected to
a high voltage external DC source (8.6V to 16.5V) the cur-
rent limit is increased so that 500mA is sourced from VCC2.
If a downstream device sinking current from VCC2 draws
more than 25mA, the input current on VCC may exceed
100mA, the USB single unit load specification for low
power devices. The LTM2884 does not enforce a 100mA
current limit for low power peripherals.
VCC2 is internally decoupled to GND2 with a 22µF capaci-
tor. Add an additional low ESR 100µF capacitor to VCC2 to
meet the VBUS downstream supply decoupling minimum
specification of 120µF when supporting device plug in.
Locate the additional 100µF capacitor adjacent to the
downstream USB connector. Additional capacitance may
not be necessary when the LTM2884 is used in a peripheral
device, or upstream hub application.
VLO and VLO2 Supplies
The VLO and VLO2 output supply pins are available for use
as low current 3.3V supplies on both sides of the isolation
barrier. They also serve as supplies for the USB interface
circuitry. An internal linear regulator maintains 3.3V on
VLO from the VBUS input supply. A separate linear regulator
maintains 3.3V on VLO2 from VCC2. The current is limited
to 10mA for external applications. Exceeding this limit
may cause degradation in the VLO or VLO2 supplies and
undesirable operation from the USB isolator. Connection of
signals ON or SPNDPWR to VLO will not cause a significant
change in the available VLO current. 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 applications on the VLO output
supply. VLO and VLO2 are protected from overcurrent and
overtemperature conditions.
For more information www.linear.com/LTM2884
2884f
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