MAX3421EEHJ(2006) Ver la hoja de datos (PDF) - Maxim Integrated
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MAX3421EEHJ Datasheet PDF : 29 Pages
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3.3V
REGULATOR
SPI
3, 4
USB
MAX3421E
INT
USB Peripheral/Host Controller
with SPI Interface
Typical Application Circuits (continued)
POWER RAIL
ASIC,
DSP,
ETC.
Two MAX3421E features make it easy to connect to
large, fast chips such as ASICs and DSPs (Figure 2).
First, the SPI interface can be clocked up to 26MHz.
Second, the VL pin and internal level translators allow
running the system interface at a lower voltage than
the 3.3V required for VCC.
Figure 2. The MAX3421E Connected to a Large Chip
3.3V
REGULATOR
LOCAL
POWER
I
S
MISO
O
INT
L
MICRO
USB
A
ASIC
MAX3421E
T
DSP
O
SCLK
R
MOSI
S
SS
LOCAL
GND
Figure 3. Optical Isolation of USB Using the MAX3421E
5V
VBUS
SWITCH
VBUS
POWER
ON/OFF
FAULT
3.3V
REGULATOR
SPI
MICRO, 3, 4
ASIC,
DSP
INT
MAX3421E
VBUS
D+
USB
USB USB
"A" D- "B" PERIPHERAL
GND
Figure 4. The MAX3421E in an Embedded Host Application
The MAX3421E provides an ideal method for electrically
isolating a USB interface (Figure 3). USB employs flow
control in which the MAX3421E automatically answers
host requests with a NAK handshake, until the micro-
processor completes its data-transfer operations over
the SPI port. This means that the SPI interface can run
at any frequency up to 26MHz. Therefore, the designer
is free to choose the interface operating frequency and
to make opto-isolator choices optimized for cost or per-
formance.
Figure 4 shows a block diagram for a system in which
the MAX3421E operates as a USB host. A USB host
supplies 5V power to the VBUS pin of the USB “A” con-
nector to power USB peripherals. A system that pro-
vides power to an external peripheral should use
protection circuitry on the power pin to prevent an
external overcurrent situation from damaging the sys-
tem. A VBUS switch, such as the MAX4789, provides
power control plus two additional features: it limits the
current delivered to the peripheral (for example to
200mA), and it indicates a fault (overcurrent) condition
to the SPI controller. Maxim offers a variety of VBUS
switches with various current limits and features.
Consult the Maxim website for details.
A 3.3V regulator (for example, the MAX6349TL) powers
the MAX3421E, and optionally the system controller. If
the system controller operates with a lower voltage, the
MAX3421E SPI and I/O interface can run at the lower
voltage by connecting the system voltage (for exam-
ple, 2.5V or 1.8V) to the MAX3421E VL pin.
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