33897 Datasheet, PDF (14/21 Pages) Freescale Semiconductor, Inc – Single Wire CAN Transceiver

FUNCTIONAL DESCRIPTION
FUNCTIONAL BLOCK DIAGRAM COMPONENTS
diode is generally used to protect the entire module from
reverse battery and should be selected accordingly.
BUS I /O
This input / output may require electrostatic discharge
(ESD) and /or EMI external circuitry. A set of components is
shown in the simplified application diagrams on page 16 of
this datasheet. The value of the capacitor should be adjusted
downward in direct proportion to the added capacitance of
the ESD or EMI circuits. The series resistance of the inductor
should be kept below 3.5 Ω to prevent its voltage drop from
significantly degrading system noise margins.
FUNCTIONAL BLOCK DIAGRAM COMPONENTS
Timer OSC
This circuit generates a 500 kHz signal to be used for
internal logic. It is the reference for some of the required
delays.
Timers
This circuit contains the timing logic used to hold the CNTL
active for the required time after the conditions for sleep
mode have been met. It is also used to keep the TXD driver
active for a period of time after it has generated a passive
level on the bus.
Mode Control
This circuit contains the control logic for the various
operating modes and conditions required for the IC.
BUS RCVR
This circuit translates the levels on the BUS terminal to a
CMOS level indicating the presence of a logic [0] or a
logic [1]. It also determines the presence of a high-voltage
wake-up (HVWU) signal that is passed to Mode Control and
Timers circuits. An analog filter is used to “de-glitch” the high-
voltage wake-up signal and prevent false exits from the sleep
mode.
TXD BUS DRVR
This circuit drives the BUS. It can drive it with the higher
voltage wake-up signals when enabled by the Mode Control
circuit. It can also provide waveshaping for reduced EMI or
not provide it for the higher data rate mode. The actual data
is received on TXD at CMOS logic levels, then translated by
this circuit to the necessary operating voltages.
Undervoltage Detect
This circuit monitors internal operating voltage to assure
proper operation of the part. If a low-voltage condition is
detected, it sends a signal to disable the BUS RCVR and
TXD BUS DRVR circuits. This prevents incorrect data from
being put on the bus or sent to the MCU.
Load Switch
The LOAD switch provides a path for an external resistor
connected to the BUS to be connected to ground. When a
loss of ground is detected, this switch is opened to prevent
the current that would normally be flowing to the ground from
the module from going back through the load resistor and
raising the bus level. The circuit is opened when the voltage
between GND and VBATT becomes too low as would be the
case if module ground were lost.
33897/A/B/C/D/T
14
Analog Integrated Circuit Device Data
Freescale Semiconductor