UJA1076A All information provided in this document is subject to legal disclaimers. © NXP B.V. 2011. All rights reserved.
Product data sheet Rev. 2 — 31 January 2011 20 of 47
NXP Semiconductors
UJA1076A
High-speed CAN core system basis chip
For short-circuit protection, a resistor needs to be connected between pins V1 and
VEXCC to allow the current to be monitored. This resistor limits the current delivered by
the external transistor. If the voltage difference between pins VEXCC and V1 reaches
V
th(act)Ilim
, the PNP current limiting activation threshold voltage, the transistor current will
not increase further.
The thermal performance of the transistor needs to be considered when calculating the
value of this resistor. A 3.3 Ω resistor was used with the BCP52-16
(NXP Semiconductors) employed during testing. Note that the selection of the transistor is
not critical. In general, any PNP transistor with a current amplification factor (β) of
between 60 and 500 can be used.
If an external PNP transistor is not used, pin VEXCC must be connected to V1 while pin
VEXCTRL can be left open.
One advantage of this scalable voltage regulator concept is that there are no PCB layout
restrictions when using the external PNP. The distance between the UJA1076A and the
external PNP doesn’t affect the stability of the regulator loop because the loop is realized
within the UJA1076A. Therefore, it is recommended that the distance between the
UJA1076A and PNP transistor be maximized for optimal thermal distribution.
The output voltage on V1 is monitored continuously and a system reset signal is
generated if an undervoltage event occurs. A system reset is generated if the voltage on
V1 falls below the undervoltage detection voltage (V
uvd
; see Table 10). The reset
threshold (90 % or 70 % of the nominal value) is set via the Reset Threshold Control bit
(RTHC) in the Int_Control register (Table 6
). In addition, an undervoltage warning (a V1UI
interrupt) will be generated at 90 % of the nominal output voltage. The status of V1 can be
read via bit V1S in the WD_and_Status register (Table 4
).
6.6.3 Voltage regulator V2
Voltage regulator V2 is reserved for the high-speed CAN transceiver, providing a 5 V
supply.
V2 can be activated and deactivated via the MC bits in the Mode_Control register
(Table 5
). An undervoltage warning (a V2UI interrupt) is generated when the output
voltage drops below 90 % of its nominal value. The status of V2 can be read via bit V2S in
the WD_and_Status register (Table 4
) in Normal mode (V2S = 1 in all other modes).
V2 can be deactivated (MC = 10) to allow the internal CAN transceiver to be supplied from
an external source or from V1. The alternative voltage source must be connected to pin
V2. All internal functions (e.g. undervoltage protection) will work normally.
6.7 CAN transceiver
The analog section of the UJA1076A CAN transceiver corresponds to that integrated into
the TJA1042/TJA1043. The transceiver is designed for high-speed (up to 1 Mbit/s) CAN
applications in the automotive industry, providing differential transmit and receive
capability to a CAN protocol controller.
6.7.1 CAN operating modes
6.7.1.1 Active mode
The CAN transceiver is in Active mode when:
