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SLLSEC4B June 2013 – August 2016 HVDA551-Q1 , HVDA553-Q1

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8 Detailed Description

8.1 Overview

The device meets or exceeds the specifications of the ISO 11898 High-Speed CAN (Controller Area Network) Physical Layer standard (transceiver). This device provides CAN transceiver functions: differential transmit capability to the bus and differential receive capability at data rates up to 1 megabit per second (Mbps). The device includes many protection features providing device and CAN network robustness.

8.2 Functional Block Diagrams

HVDA551-Q1 HVDA553-Q1 fbd_hvda551_SLLSEC4.gif

Figure 13. HVDA551 Block Diagram

HVDA551-Q1 HVDA553-Q1 fbd_hvda553_llsec4.gif

Figure 14. HVDA553 Block Diagram

8.3 Feature Description

8.3.1 Digital Inputs and Outputs

The HVDA551 device has an I/O supply voltage input pin (V_IO) to ratiometrically level shift the digital logic input and output levels with respect to V_IO for compatibility with protocol controllers having I/O supply voltages between 3 V and 5.33 V.

The HVDA553 devices have a single V_CC supply (5 V). The digital logic input and output levels for these devices are with respect to V_CC for compatibility with protocol controllers having I/O supply voltages between 4.68 V and 5.33 V.

8.3.2 Using the HVDA553 With Split Termination

The SPLIT pin voltage output provides 0.5 × V_CC in normal mode. The circuit may be used by the application to stabilize the common-mode voltage of the bus by connecting it to the center tap of split termination for the CAN network (see Figure 15 and Figure 23). This pin provides a stabilizing recessive voltage drive to offset leakage currents of unpowered transceivers or other bias imbalances that might bring the network common-mode voltage away from 0.5 × V_CC. Using this feature in a CAN network improves electromagnetic emissions behavior of the network by eliminating fluctuations in the bus common-mode voltage levels at the start of message transmissions.

HVDA551-Q1 HVDA553-Q1 SPLIT_PIN_FIGURE.gif

Figure 15. SPLIT Pin Circuitry and Application

8.3.3 Protection Features

8.3.3.1 TXD Dominant State Time Out

During normal mode, the only mode where the CAN driver is active, the TXD dominant time-out circuit prevents the transceiver from blocking network communication in the event of a hardware or software failure where TXD is held dominant longer than the time-out period t_(DOM). The dominant time-out circuit is triggered by a falling edge on TXD. If no rising edge is seen before the time-out constant of the circuit expires (t_(DOM)) the CAN bus driver is disabled, freeing the bus for communication between other network nodes. The CAN driver is re-activated when a recessive signal is seen on the TXD pin, thus clearing the dominant-state time-out. The CAN bus pins are biased to the recessive level during a TXD dominant-state time-out.

NOTE

The maximum dominant TXD time allowed by the TXD dominant-state time-out limits the minimum possible data rate of the devices.

The CAN protocol allows a maximum of eleven successive dominant bits (on TXD) for the worst case, where five successive dominant bits are followed immediately by an error frame. This, along with the t_(DOM) minimum, limits the minimum bit rate. The minimum bit rate may be calculated by Equation 1:

Equation 1. Minimum Bit Rate = 11 / t_(DOM)

8.3.3.2 Thermal Shutdown

If the junction temperature of the device exceeds the thermal shutdown threshold, the device turns off the CAN driver circuits. This condition is cleared once the temperature drops below the thermal shutdown temperature of the device. The CAN bus pins are biased to the recessive level during a thermal shutdown.

8.3.3.3 Undervoltage Lockout or Unpowered Device

Both of the supply pins have undervoltage detection, which places the device in forced standby mode to protect the bus during an undervoltage event on either the V_CC or V_IO supply pins. If V_IO is undervoltage, the RXD pin is forced to the high-impedance state and the device does not pass any wake-up signals from the bus to the RXD pin. Because the device is placed into forced standby mode, the CAN bus pins have a common-mode bias to ground, protecting the CAN network; see Figure 16 and Figure 17.

The device is designed to be an ideal passive load to the CAN bus if it is unpowered. The bus pins (CANH, CANL) have extremely low leakage currents when the device is unpowered, so they do not load down the bus but rather be a no-load. This is critical, especially if some nodes of the network are unpowered while the rest of the network remains in operation.

NOTE

Once an undervoltage condition is cleared and V_CC and V_IO have returned to valid levels, the device typically requires 300 µs to transition to normal operation.

Table 2. Undervoltage Protection

DEVICE	V_CC	V_IO	DEVICE STATE	BUS	RXD
Both devices	Bad	Good	Forced standby mode	Common mode bias to GND⁽¹⁾	Mirrors bus state through wake-up filter⁽²⁾
	Good	Bad	Forced standby mode⁽³⁾	Common mode bias to GND⁽¹⁾	High Z
	Unpowered		Unpowered	No load	High Z

(1) See Figure 16 and Figure 17 for common-mode bias information.

(2) See Figure 18 and Figure 19 for operation of the low-power wake-up receiver and bus monitor for RXD wake-up request behavior and Table 5 for the wake-up receiver threshold levels.

(3) When V_IO is undervoltage, the device is forced into standby mode with respect to the CAN bus, because there is not a valid digital reference to determine the digital I/O states or power the wake-up receiver.

8.3.3.4 Floating Pins

The device has integrated pullups and pulldowns on critical pins to place the device into known states if the pins float. The TXD and STB pins on the HVDA551 are pulled up to V_IO. This forces a recessive input level on TXD in the case of a floating TXD pin and prevents the device from entering into the low-power standby mode if the STB pin floats. In the case of the HVDA553 both the TXD and STB pins are pulled up to V_CC, which has the same effect.

8.3.3.5 CAN Bus Short-Circuit Current Limiting

The device has several protection features that limit the short-circuit current when a CAN bus line is shorted. These include CAN driver-current limiting (dominant and recessive) and TXD dominant-state time-out to prevent continuously driving dominant. During CAN communication, the bus switches between dominant and recessive states; thus, the short-circuit current may be viewed either as the current during each bus state or as a DC average current. For system current and power considerations in termination resistance and common-mode choke ratings, the average short-circuit current must be used. The device has TXD dominant-state time-out, which prevents permanently having the higher short-circuit current of dominant state. The CAN protocol also has forced state changes and recessive bits such as bit stuffing, control fields, and interframe space. These ensure there is a minimum recessive amount of time on the bus even if the data field contains a high percentage of dominant bits.

NOTE

The short-circuit current of the bus depends on the ratio of recessive to dominant bits and their respective short-circuit currents.

The average short-circuit current may be calculated with Equation 2:

Equation 2. I_OS(AVG) = %Transmit × [(%REC_Bits × I_{OS(SS)_REC}) + (%DOM_Bits × I_{OS(SS)_DOM})] + [%Receive × I_{OS(SS)_REC}]

where

I_OS(AVG) is the average short-circuit current
%Transmit is the percentage the node is transmitting CAN messages
%Receive is the percentage the node is receiving CAN messages
%REC_Bits is the percentage of recessive bits in the transmitted CAN messages
%DOM_Bits is the percentage of dominant bits in the transmitted CAN messages
I_{OS(SS)_REC} is the recessive steady-state, short-circuit current
I_{OS(SS)_DOM} is the dominant steady-state, short-circuit current

8.4 Device Functional Modes

These devices have two main operating modes: normal mode and standby mode. Table 3 lists these modes in detail. Operating mode selection is made through the STB input pin.

Table 3. Operating Modes

DEVICE	STB	MODE	DRIVER	RECEIVER	RXD Pin
All devices	LOW	Normal mode	Enabled (On)	Enabled (On)	Mirrors bus state⁽¹⁾
All devices	HIGH	Standby mode (RXD wake-up request)	Disabled (Off)	Low-power wake-up receiver and bus monitor enabled	Mirrors bus state through wake-up filter⁽²⁾

(1) Mirrors bus state: LOW if CAN bus is dominant, HIGH if CAN bus is recessive.

(2) See Figure 18 and Figure 19 for operation of the low-power wake-up receiver and bus monitor for RXD wake-up request behavior and Table 5 for the wake-up receiver threshold levels.

8.4.1 Bus States by Mode

The CAN bus has three valid states during powered operation, depending on the mode of the device. In normal mode, the bus may be dominant (logic LOW) where the bus lines are driven differentially apart, or or the bus may be recessive (logic HIGH) where the bus lines are biased to V_CC / 2 through the high-ohmic internal input resistors R_IN of the receiver. The third state is low-power standby mode where the bus lines are biased to GND through the high-ohmic internal input resistors R_IN of the receiver.

HVDA551-Q1 HVDA553-Q1 bus_states_physical_bit_lls804.gif

Figure 16. Bus States (Physical Bit Representation)

HVDA551-Q1 HVDA553-Q1 simplified_common_mode_bias_lls804.gif

Figure 17. Simplified Common-Mode Bias and Receiver Implementation

8.4.2 Normal Mode

This is the normal operating mode of the device. Normal mode is selected by setting STB low. The CAN driver and receiver are fully operational and CAN communication is bidirectional. The driver is translating a digital input on TXD to a differential output on CANH and CANL. The receiver is translating the differential signal from CANH and CANL to a digital output on RXD. In recessive state, the CAN bus pins (CANH and CANL) are biased to 0.5 × V_CC. In dominant state, the bus pins are driven differentially apart. Logic high is equivalent to recessive on the bus, and logic low is equivalent to a dominant (differential) signal on the bus.

8.4.3 Standby Mode With RXD Wake-Up Request

This is the low-power mode of the device. Standby mode is selected by setting STB high. The CAN driver and main receiver are turned off and bidirectional CAN communication is not possible. The low-power receiver and bus monitor, both supplied through the V_IO supply, are enabled to allow for RXD wake-up requests through the CAN bus. The V_CC (5-V) supply may be turned off for additional power savings at the system level. A wake-up request is output to RXD (driven low) for any dominant bus transmissions longer than the filter time t_BUS. The local protocol controller (MCU) must monitor RXD for transitions and then reactivate the device to normal mode based on the wake-up request. The 5-V (V_CC) supply must be reactivated by the local protocol controller to resume normal mode if it has been turned off for low-power standby operation. The CAN bus pins are weakly pulled to GND, see Figure 16 and Figure 17.

8.4.3.1 RXD Wake-Up Request Lockout for Bus-Stuck Dominant Fault (HVDA551)

If the bus has a fault condition where it is stuck dominant while the HVDA551 is placed into standby mode through the STB pin, the device locks out the RXD wake-up request until the fault has been removed to prevent false wake-up signals in the system.

HVDA551-Q1 HVDA553-Q1 ent_stby_rec_lls804.gif

Figure 18. HVDA551 RXD Wake-Up Request With No Bus Fault Condition

HVDA551-Q1 HVDA553-Q1 ent_stby_dom_lls804.gif

Figure 19. HVDA551 RXD Wake-Up Request Lockout During Bus Dominant Fault Condition

8.4.4 Driver and Receiver Function Tables

Table 4 shows the behavior of devices when in driver mode.

Table 4. Driver Function Table

DEVICE	INPUTS		OUTPUTS		DRIVEN BUS STATE
DEVICE	STB / S⁽¹⁾	TXD⁽¹⁾	CANH⁽¹⁾	CANL⁽¹⁾	DRIVEN BUS STATE
Both devices	L	L	H	L	Dominant
	L	H	Z	Z	Recessive
	L	Open	Z	Z	Recessive
HVDA551, HVDA553⁽²⁾	H	X	Y	Y	Recessive

(1) H = high level, L = low level, X = irrelevant, Y = common-mode bias to GND, Z = common-mode bias to V_CC / 2. See Figure 16 and Figure 17 for common-mode bias information.

(2) HVDA551 and HVDA553 have internal pullup to V_IO on the STB pin. If the STB pin is open, the pin is pulled high and the device is in standby mode.

Table 5. Receiver Function Table

DEVICE MODE	CAN DIFFERENTIAL INPUTS V_ID = V(CANH) – V(CANL)	BUS STATE	RXD PIN⁽¹⁾
Standby with RXD wake-up request (HVDA551, HVDA553)⁽²⁾	V_ID ≥ 1.15 V	DOMINANT	L
	0.4 V < V_ID < 1.15 V	?	?
	V_ID ≤ 0.4 V	RECESSIVE	H
NORMAL	V_ID ≥ 0.9 V	DOMINANT	L
	0.5 V < V_ID < 0.9 V	?	?
	V_ID ≤ 0.5 V	RECESSIVE	H
ANY	Open	N/A	H

(1) H = high level, L = low level, X = irrelevant, ? = indeterminate.

(2) While STB is high (standby mode) the RXD output of the HVDA551 functions according to the levels above and the wake-up conditions shown in Figure 18 and Figure 19.