UJA1079A 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 21 of 46
NXP Semiconductors
UJA1079A
LIN 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 UJA1079A and the
external PNP doesn’t affect the stability of the regulator loop because the loop is realized
within the UJA1079A. Therefore, it is recommended that the distance between the
UJA1079A 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
).
6.7 LIN transceiver
The analog sections of the UJA1079A LIN transceiver are derived from those integrated
into the TJA1021. Unlike the TJA1021 however, the UJA1079A does not include an
internal slave termination resistor. Therefore, external termination resistors need to be
connected in both master and slave applications (see Figure 9
and Figure 10).
The transceiver is the interface between the LIN master/slave protocol controller and the
physical bus in a LIN. It is primarily intended for in-vehicle sub-networks using baud rates
from 1 kBd up to 20 kBd and is LIN 2.0/LIN 2.1/SAE J2602 compliant.
