9
FN6342.1
December 7, 2006
The USB switches are active (turned ON) whenever the
V
BUS
voltage is to V
DD
+ 0.8V. V
BUS
is internally pulled
low, so when V
BUS
is floating, the USB switches are OFF.
Note: Whenever the USB switches are ON the audio drivers
of the CODEC need to be at AC or DC ground or floating to
keep from interfering with the data transmission.
ISL54205A Operation
The discussion that follows will discuss using the ISL54205A
in the typical application shown in the block diagram on
page 8.
LOGIC CONTROL
The state of the ISL54205A device is determined by the
voltage at the VBUS pin (pin 2) and the CTRL pin (pin 10).
Refer to truth-table on page 2 of data sheet.
The VBUS pin and CTRL pin are internally pulled low
through 4M
resistors to ground and can be left floating. The
CTRL control pin is only active when V
BUS
is logic “0”.
Logic control voltage levels:
V
BUS
= Logic “0” (Low) when V
BUS
V
DD
+ 0.2V or
Floating.
V
BUS
= Logic “1” (High) when V
BUS
V
DD
+ 0.8V
CTRL = Logic “0” (Low) when
0.5V or floating.
CTRL = Logic “1” (High) when
1.4V
Audio Mode
If the VBUS pin = Logic “0” and CTRL pin = Logic “1,” the
part will be in the Audio mode. In Audio mode the L (left) and
R (right) 3 audio switches are ON and the D- and D+ 5
switches are OFF (high impedance). In a typical application,
V
DD
will be in the range of 2.7V to 3.6V and will be
connected to the battery or LDO of the MP3 player or cell
phone. When a headphone is plugged into the common
connector, nothing gets connected at the VBUS pin (it is
floating) and as long as the CTRL = Logic “1,” the
ISL54205A part remains in the audio mode and the audio
drivers of the player can drive the headphones and play
music.
USB Mode
If the VBUS pin = Logic “1” and CTRL pin = Logic “0” or
Logic “1,” the part will go into USB mode. In USB mode, the
D- and D+ 5 switches are ON and the L and R 3 audio
switches are OFF (high impedance). When a USB cable
from a computer or USB hub is connected at the common
connector, the voltage at the VBUS pin will be driven to be in
the range of 4.4V to 5.25V. The ISL54205A part will go into
the USB mode. In USB mode, the computer or USB hub
transceiver and the MP3 player or cell phone USB
transceiver are connected and digital data will be able to be
transmitted back and forth.
When the USB cable is disconnected, the ISL54205A
automatically turns the D+ and D- switches OFF.
Low Power Mode
If the VBUS pin = Logic “0” and CTRL pin = Logic “0,” the
part will be in the Low Power mode. In the Low Power mode,
the audio switches and the USB switches are OFF (high
impedance). In this state, the device draws typically 1nA of
current.
EXTERNAL VBUS SERIES RESISTOR
The ISL54205A contains a clamp circuit between VBUS and
VDD. Whenever the VBUS voltage is greater than the VDD
voltage by more than 2.55V, current will flow through this
clamp circuitry into the V
DD
power supply bus.
During normal USB operation, V
DD
is in the range of 2.7V to
3.6V and V
BUS
is in the range of 4.4V to 5.25V. The clamp
circuit is not active and no current will flow through the clamp
into the VDD supply.
In a USB application, the situation can exist where the V
BUS
voltage from the computer could be applied at the VBUS pin
before the V
DD
voltage is up to its normal operating voltage
range and current will flow through the clamp into the V
DD
power supply bus. This current could be quite high when
V
DD
is OFF or at 0V and could potentially damage other
components connected in the circuit. In the application
circuit, a 22k
resistor has been put in series with the VBUS
pin to limit the current to a safe level during this situation.
It is recommended that a current limiting resistor in the range
of 10k
to 50k be connected in series with the VBUS pin. It
will have minimal impact on the logic level at the VBUS pin
during normal USB operation and protect the circuit during
the time VBUS is present before VDD is up to its normal
operating voltage.
Note: No external resistor is required in applications where
V
BUS
will not exceed V
DD
by more than 2.55V.
POWER
The power supply connected at VDD (pin 1) provides power
to the ISL54205A part. Its voltage should be kept in the
range of 2.7V to 3.6V when used in a USB/Audio application
to ensure you get proper switching when the V
BUS
voltage is
at its lower limit of 4.4V.
ISL54205A
