Micrel, Inc.
MIC2555
Pin Descriptions
VTRM can be used to supply small amounts of current to
other system functions, typically 3 mA or less. However,
trying to source more current can reduce output drive on
D+, D- by stealing current from the differential driver.
VBAT
This pin is an input, and supplies power to the
transceiver. Transceiver typical operational voltages are
between 3.0 V ≤ VBAT ≤ 4.5V and 1.6V < VDD_LGC < VBAT.
ID
ID detects the arrival or departure of a peripheral device,
and differentiates between USB and non-USB devices.
To accomplish this, ID is pulled-up by a resistance of
approximately 100 kꢀ connected to VBAT and the voltage
at ID monitored by a set of comparators. When no
device is present, ID is pulled high and NO DEVICE
condition is reported. When a Mini-A plug is inserted into
the system’s Mini-AB receptacle, ID is connected to
ground by the Mini-A plug, which triggers the MIC2555
to indicate a USB device is present.
VDD_LGC
This input is used to set the logic thresholds of the
following logic signals:
•
•
•
•
•
•
•
•
DAT_VP_RX
SE0_VM_TX
RCV
OE_INT/
INT/
Non-USB peripherals use a modified Mini-A plug or non-
standard cable assembly with a resistor connected
between ID and ground. When connected, this forms a
resistor divider such that a voltage of approximately ½
ADR0
RESET/
GPIO
VBAT appears at MIC2555’s ID pin, indicating a non-USB
Important Note:
device is present.
VDD_LGC can be at a voltage less than or equal to VBAT
,
Additionally, ID can be used to signal non-USB devices.
This is accomplished by grounding ID through a low
value resistor (~ 1 kꢀ), dropping the ID voltage from ½
VBAT to nearly zero, which can be detected by the
attached device. This switch is activated by the
id_gnd_out bit in Control Register 2.
but never higher than VBAT. Doing so will forward bias
internal pad protection diodes and current will flow from
VDD_LGC to VBAT. For this reason, systems should not
allow VBAT to go to zero while VDD_LGC remains powered.
This condition may damage the MIC2555, and could put
a severe load on VDD_LGC as it attempts to power the
MIC2555 and all other circuits attached to the VBAT line.
C-, C+, C++
C-, C+ and C++ are the capacitors required for charge
pump operation. C- and C+ are the connections to the
‘flying’ capacitor, which creates the pumping effect. C++
is the reservoir capacitor that stores the 5V supplied to
VBUS
This pin functions as both an input to, and output from,
the transceiver. Unlike standard USB transceivers,
however, the MIC2555 always derives its operating
power from VBAT and never from VBUS. The MIC2555 will
supply power to VBUS when acting as a host device and
when petitioning another OTG, capable device to
become the host. To do so the vbus_chrg bit is asserted.
To power VBUS, as a host device, the vbus_drv bit is
asserted. The difference between these two controls is
vbus_chg applies VTRM (3.3V) to VBUS, where as
vbus_driv uses the 5V charge pump output. While VTRM
is sufficient for signaling purposes, it does not meet the
VBUS when vbus_drv is asserted.
Because the input source is a low voltage and the
charge pump’s regulator is set to limit VOUT to 5V, these
capacitors need only be rated at 6 VDC, which helps
reduce physical size and cost.
GND_A, GND_D
MIC2555 uses separate ground lines within the chip to
isolate digital noise from analog signals. Ultimately,
these two grounds need to be tied together. This is best
done by having both grounds return separately to the
power source and join at the bypass capacitor.
4.4V minimum for VBUS
.
VTRM
VTRM supplies a regulated 3.3V to the D+, D- output
drivers, pull-up resistors and other circuitry internal to the
MIC2555. A small filter capacitor is required to insure the
regulator remains stable under all operating conditions.
A good quality 1µF capacitor is sufficient for this
purpose.
RESET/
System reset. Returns all control register bits to their
default settings. MIC2555 is not equipped with an
internal power-on reset generator, and thus relies upon
the system for its reset at power up.
M9999-121406
December 2006
17
MICREL [ MICREL SEMICONDUCTOR ]
