MIC2550_11 Datasheet, PDF (8/11 Pages) Micrel Semiconductor – Universal Serial Bus Transceiver

MIC2550
Applications Information
The MIC2550 is designed to provide USB connectivity in
mobile systems where system supply voltages are not avail-
able to satisfy USB requirements. The MIC2550 can operate
down to supply voltages of 2.5V and still meet USB physical
layer specifications. As shown in the system diagram, the
MIC2550 takes advantage of USB’s supply voltage, V_BUS, to
operate the transceiver. The system voltage, V_IF, is used to
set the reference voltage used by the digital I/O lines (VP, VM,
RCV, OE#, SPD, and SUS pins) interfacing to the system.
Internal circuitry provides translation between the USB and
system voltage domains. V_IF will typically be the main supply
voltage rail for the system.
In addition, a 3.3V, 10% termination supply voltage, V_TRM, is
provided to support speed selection. A 0.47µF (minimum)
capacitor from V_TRM to ground is required to ensure stability.
As shown in the typical application diagram, a 1µF capacitor
is recommended. A 1.5K resistor is required between this pin
and the D+ or D– lines to respectively specify full-speed or
low-speed operation.
Power Supply Configurations
V_IF/V_BUS Switched
When the V_BUS input pin is pulled to ground a low impedance
path between V_IF and V_BUS can cause a high current flow
from V_IF to V_BUS thereby damaging the MIC2550. This issue
can arise in systems where V_BUS is driven from a power
supply that can be switched off such as in the case of a
desktop PC. Adding a Schottky diode, such as the ZHCS1000
by Zetex, in series with V_BUS will prevent any current flow
during this condition. A solution is shown in Figure 8 below.
USB Device
Power Controller
MIC2550
VIF VBUS
D1
ZHCS1000 or
equivalent
*(Optional)
VBUS
1µF min
Note: *(Optional) See Text - Power Supply Configurations
Figure 8. Solution to V_IF/V_BUS Switching
I/O Interface Using 3.3V
In systems where the I/O interface utilizes a 3.3V USB
controller, an alternate solution is shown in Figure 9. This
configuration has the advantage over Figure 8, in that no
extra components are needed. Ensure that the load on V_TRM
does not exceed 1mA total.
3.3V
MIC2550
V_DD
VIF
VBUS
USB
Controller
I/O
V_P/V_M/
VTRM
RCV/OE#
VBUS
Micrel, Inc.
Internal 3.3V Source
If the device is self-powered and has 3.3V available, the
circuit in Figure 10 is yet another power supply configuration
option. In this configuration, the internal regulator is disabled
and the 3.3V source and not V_BUS powers the entire chip.
MIC2550
VIF
VBUS
3.3V
VTRM
Figure 10. Powering Chip
from Internal 3.3V Source
Suspend
When the suspend pin (SUS) is high, power consumption is
reduced to a minimum. V_TRM is not disabled. RCV, V_P and V_M
are still functional to enable the device to detect USB activity.
For minimal current consumption in suspend mode, it is
recommended that OE# = 1.
External ESD Protection
The use of ESD transient protection devices is not required
for operation, but is recommended. We recommend the
following devices or the equivalent:
Cooper Electronics Technologies (www.cooperet.com)
41206ESDA SurgX^®
0805ESDA SurgX^®
Littelfuse (www.littelfuse.com)
V0402MHS05
SP0503BAHT
Non-Multiplexed Bus
To save pin count for the USB logic controller interface, the
MIC2550 was designed with V_P and V_M as bidirectional pins.
To interface the MIC2550 with a non-multiplexed data bus,
resistors can be used for low cost isolation as shown in
Figure 11.
USB Logic
Controller
(SIE)
MIC2550
V_P
V_P
10k
V_PO
V_M
V_M
10k
V_MO
Figure 11. MIC2550 Interface to
Non-Multiplexed Data Bus
Figure 9. I/O Interface Uses 3.3V
M9999-031805
8
March 2005