
MAX3420E in a Bus-Powered Application
Figure 16 depicts the MAX3420E in a peripheral device
that is powered by VBUS. This configuration is advanta-
geous because it requires no external power supply.
VBUS is specified from 4.75V to 5.25V, so a 3.3V regu-
lator is required to power the MAX3420E. This diagram
assumes that the microprocessor is powered by 3.3V
as well, so the VL pin (logic-level reference voltage) is
connected to VCC. Therefore, the GPIO (general-pur-
pose inputs/outputs) are referenced to 3.3V.
USB is a hot-plug system (VBUS is hot when the device
is plugged in), so it is good design practice to use a
power-on reset circuit to provide a clean reset to the
system when the device is plugged in. The MAX6349TL
serves as an excellent USB regulator, since it has very
low quiescent current and a POR circuit built in.
Because this design is bus powered, it is not necessary
to test for the presence of VBUS. In this case, the bus
voltage-detection input, VBCOMP, makes an excellent
general-purpose input when pulled up to VL. The
VBCOMP input has two interrupts associated with it,
VBUSIRQ and NOVBUSIRQ. These interrupts can detect
both edges of any transitions on the VBCOMP input.
The configuration in Figure 16 shows the SPI interface
using the maximum number of SPI interface pins. The
data pins, MOSI and MISO, are separate, and the
MAX3420E supplies an interrupt signal through the INT
output pin to the P to notify the P when its attention
is required.
MAX3420E in a Self-Powered Application
Figure 17 shows a self-powered design in which the P
has its own power source. This is a common configura-
tion in battery-powered handheld devices. Figure 17
also illustrates the SPI interfacing with the minimum
number of pins. This is achieved by using a single bidi-
rectional data line and no interrupt pin connection. The
MAX3420E register bit, FDUPSPI, configures the SPI
interface for bidirectional operation.
Although Figure 17 shows VL = VCC, if the microcon-
troller uses a different interface voltage (1.71V to 3.6V)
this reference voltage can be connected to VL. Figure 17
shows a connection from the MAX3420E GPX output to
the microcontroller. GPX can be programmed (see Table
3) to connect the output of the internal VBUS comparator
to the GPX output. This enables the microprocessor to
detect a USB plug-in event even if the MAX3420E is put
into its power-down state.
USB Peripheral Controller
with SPI Interface
MAX3420E
VCC
VL
XI
XO
INT
MOSI
MISO
SCLK
RES
D+
D-
D+
D-
VBCOMP
SS
0.1
μF
10k
Ω
GPI
GND
VBUS
33
Ω
33
Ω
1.0
μF
CERAMIC
CXI
CXO
12MHz
3.3V
REGULATOR
MAX6349TL
μP
4
USB
"B" CONNECTOR
GND
GPIN GPOUT
4.7
μF
Figure 16. MAX3420E in a Bus-Powered Application
MAX3420E
Maxim Integrated
19