10
FN6557.0
September 25, 2007
minimum edge and phase distortion to meet USB 2.0 high
speed signal quality specifications. See high-speed eye
diagram in the “Typical Performance Curves” on
page 12,
The USB switches can also pass USB full-speed signals
(12Mbps) with minimal distortion and meet all the USB
requirements for USB 2.0 full-speed signaling. See
full-speed eye diagram in the “Typical Performance Curves”
The maximum signal range for the USB switches is from
-1.5V to VDD. The signal voltage at D- and D+ should not be
allowed to exceed the VDD voltage rail or go below ground
by more than -1.5V.
The USB switches are active (turned ON) whenever the
VBUS voltage is ≥ to VDD + 0.8V. VBUS is internally pulled
low, so when VBUS 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.
ISL54205B Operation
The discussion that follows will discuss using the ISL54205B
LOGIC CONTROL
The state of the ISL54205B device is determined by the
voltage at the VBUS pin (pin 2) and the CTRL pin (pin 10).
The VBUS pin and CTRL pin are internally pulled low
through 4MΩ resistors to ground and can be left floating. The
CTRL control pin is only active when VBUS is logic “0”.
Logic Control Voltage Levels:
VBUS = Logic “0” (Low) when VBUS ≤ VDD + 0.2V or
Floating.
VBUS = Logic “1” (High) when VBUS ≥ VDD + 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
Ω audio switches are ON and the D- and D+ 5Ω
switches are OFF (high impedance). In a typical application,
VDD 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
ISL54205B 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
Ω switches are ON and the L and R 3Ω 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 ISL54205B 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 ISL54205B
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 ISL54205B 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 VDD power supply bus.
During normal USB operation, VDD is in the range of 2.7V to
3.6V and VBUS 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 VBUS
voltage from the computer could be applied at the VBUS pin
before the VDD voltage is up to its normal operating voltage
range and current will flow through the clamp into the VDD
power supply bus. This current could be quite high when VDD is
OFF or at 0V and could potentially damage other components
connected in the circuit. In the application circuit, a 22kΩ
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
10kΩ to 50kΩ 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
VBUS will not exceed VDD by more than 2.55V.
POWER
The power supply connected at VDD (pin 1) provides power to
the ISL54205B 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 VBUS voltage is at its
lower limit of 4.4V.
ISL54205B