LMD18400 Datasheet, PDF (15/26 Pages) National Semiconductor (TI) – Quad High Side Driver

LMD18400
www.ti.com
SNVS094C – JUNE 1996 – REVISED APRIL 2013
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 designing “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-temperature indication will be reported in the diagnostic data stream.
Figure 26 illustrates the use of pin 17 as both an output thermal shutdown flag and as an input to shut down only
the switches. Directly tying pin 17 to +5V will prevent the internal 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 detect open or shorted load connections. Figure 27
illustrates the detection circuit used with each of the drivers.
Figure 27. 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
kΩ 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 indicated for that switch. The maximum
load resistance that will not generate an Open Load Error when a switch is OFF can be found by:
(2)
To make this Open Load Error threshold more sensible, 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 indicate 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 shorted 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:
(3)
Figure 28 indicates the range of load resistance for normal operation, open load, and shorted load or power limit
indication.
Copyright © 1996–2013, Texas Instruments Incorporated
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