MIC2550A Datasheet, PDF (8/11 Pages) Micrel Semiconductor – Universal Serial Bus Transceiver

MIC2550A
Applications Information
The MIC2550A is designed to provide USB connectivity in
mobile systems where system supply voltages are not avail-
able to satisfy USB requirements. The MIC2550A can oper-
ate down to supply voltages of 2.5V and still meet USB
physical layer specifications. As shown in the system dia-
gram, the MIC2550A takes advantage of USB’s supply
voltage, V_BUS, to operate the transceiver. The system volt-
age, 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 transla-
tion 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.
A 1.5K resistor is required between this pin and the D+ or D–
lines to respectively specify full-speed or low-speed opera-
tion.
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 MIC2550A. 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.
If the V_IF source is current limited to less than 50mA, then
diode D1 is not necessary.
USB Device
Power Controller
MIC2550
VIF VBUS
D1
ZHCS1000 or
equivalent
*(Optional)
VBUS
1µF min
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.
MIC2550A
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, and SPD = 0.
Speed
The speed pin (SPD) sets D+/D– output edge rates by increas-
ing or decreasing biasing current sources within the output
drivers. For low speed, SPD = 0. For full speed, SPD = 1. By
setting SPD = 0 during idle periods, in conjunction with suspend
(SUS), the lowest quiescent current can be obtained. However,
designers must provide a 300ns delay between changing SPD
from 0 to 1 and transmission of data at full speed. This delay
ensures the output drivers have arrived at their proper operating
conditions. Failure to do so can result in leading edge distortion
on the first few data bits transmitted.
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^®
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
Figure 9. I/O Interface Using 3.3V
Littelfuse (www.littlefuse.com)
V0402MHS05
SP0503BAHT
Non-multiplexed Bus
To save pin count for the USB logic controller interface, the
MIC2550A was designed with V_P and V_M as bidirectional
pins. To interface the MIC2550A 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. MIC2550A Interface to
Non-multiplexed Data Bus
M9999-031805
8
March 2005