LMD18400 Datasheet, PDF (11/18 Pages) National Semiconductor (TI) – Quad High Side Driver

Applications Information (Continued)
The Error Flag output pin is an open drain transistor which
requires a pull-up resistor to a positive voltage of up to 16V
Typically this pull-up is to the same 5V supply which is bias-
ing the Enable input and any other external logic circuitry
The Error Flag pins of several LMD18400 packages can be
connected together with just one pull-up resistor to provide
an all-encompassing general system error indication Upon
detection of an error each device could then be polled for
diagnostic information to determine the source of the fault
condition
A second direct output error flag is for an indication of Ther-
mal Shutdown (pin 17) This active low flag provides an im-
mediate indication that the die temperature has reached
a170 C and that the drive to all four switches has been
removed This output is pulled up to the internal 5 1V logic
regulator through a small (5 mA) current source so use of a
buffer on this pin is recommended
TL H 11026 – 18
FIGURE 9 Thermal Shutdown Flag and Shutdown Input
A useful feature of pin 17 is that it can also be used as a
shutdown input Driving this pin low immediately switches all
of the drivers OFF just the same as if thermal shutdown
temperatures has been reached yet all of the control logic
and diagnostic circuits remain active This is useful in de-
signing ‘‘fail-safe’’ systems where the loads can be disabled
under any sort of externally detected system fault condition
The diagnostic logic however does not distinguish between
normal thermal shutdown or the fact that pin 17 has been
driven low As such various switch errors and an over-tem-
perature indication will be reported in the diagnostic data
stream
Figure 9 illustrates the use of pin 17 as both an output ther-
mal shutdown flag and as an input to shut down only the
switches Directly tying pin 17 to a5V will prevent the inter-
nal thermal shutdown circuitry from disabling the switches
For reliability purposes however this is not recommended as
there will then be no limit to the maximum die temperature
Refer to the Truth Table for a summary of the action of
these direct-output error flags
LOAD ERROR DETECTION
An important feature of the LMD18400 is the ability to de-
tect open or shorted load connections Figure 10 illustrates
the detection circuit used with each of the drivers
TL H 11026 – 19
FIGURE 10 Detection Circuitry
for Open Shorted Loads
A voltage comparator monitors the voltage to the load and
compares it to a fixed 4 1V reference level When a switch is
OFF the ground referenced load should have no voltage
across it Under this condition an internal 50 kX resistor
connected to V_CC will provide a small amount of current to
the load If the load resistance is large enough to create a
voltage greater than 4 1V an Open Load Error will be indi-
cated for that switch The maximum load resistance that will
not generate an Open Load Error when a switch is OFF can
be found by
4 1V
R_Max e _{VCC b 4 6V} c 50 kX for no Open Load Indication
To make this Open Load Error threshold more sensitive an
external pull-up resistor can be added from the output to the
V_CC supply
Also when a switch is commanded OFF should the load be
shorted to the V_CC supply this same circuitry will again indi-
cate an error
When a switch is commanded ON the load is expected to
have a voltage across it that approaches the V_CC potential
If the output voltage is less than the 4 1V threshold an error
will again be reported indicating that the load is either short-
ed to ground or that the driver is in power limit and not able
to pull the output voltage any closer to V_CC The minimum
load resistance that will not generate a Shorted Load Error
when a switch is ON can be found by
R_Min
e
4
1V
(V_CC b
15W
4
1V)
for no Shorted Load Error
11
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