LMD18200T Datasheet, PDF (8/14 Pages) National Semiconductor (TI) – 3A, 55V H-Bridge

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FIGURE 4. Transitions in Brake, Direction, or PWM Must Be Separated By At Least 1 µsec
USING THE CURRENT SENSE OUTPUT
The CURRENT SENSE output (pin 8) has a sensitivity of 377
μA per ampere of output current. For optimal accuracy and
linearity of this signal, the value of voltage generating resistor
between pin 8 and ground should be chosen to limit the max-
imum voltage developed at pin 8 to 5V, or less. The maximum
voltage compliance is 12V.
It should be noted that the recirculating currents (free wheel-
ing currents) are ignored by the current sense circuitry. There-
fore, only the currents in the upper sourcing outputs are
sensed.
USING THE THERMAL WARNING FLAG
The THERMAL FLAG output (pin 9) is an open collector tran-
sistor. This permits a wired OR connection of thermal warning
flag outputs from multiple LMD18200's, and allows the user
to set the logic high level of the output signal swing to match
system requirements. This output typically drives the interrupt
input of a system controller. The interrupt service routine
would then be designed to take appropriate steps, such as
reducing load currents or initiating an orderly system shut-
down. The maximum voltage compliance on the flag pin is
12V.
SUPPLY BYPASSING
During switching transitions the levels of fast current changes
experienced may cause troublesome voltage transients
across system stray inductance.
It is normally necessary to bypass the supply rail with a high
quality capacitor(s) connected as close as possible to the
V_S Power Supply (Pin 6) and GROUND (Pin 7). A 1 μF high-
frequency ceramic capacitor is recommended. Care should
be taken to limit the transients on the supply pin below the
Absolute Maximum Rating of the device. When operating the
chip at supply voltages above 40V a voltage suppressor (tran-
sorb) such as P6KE62A is recommended from supply to
ground. Typically the ceramic capacitor can be eliminated in
the presence of the voltage suppressor. Note that when driv-
ing high load currents a greater amount of supply bypass
capacitance (in general at least 100 μF per Amp of load cur-
rent) is required to absorb the recirculating currents of the
inductive loads.
CURRENT LIMITING
Current limiting protection circuitry has been incorporated into
the design of the LMD18200. With any power device it is im-
portant to consider the effects of the substantial surge cur-
rents through the device that may occur as a result of shorted
loads. The protection circuitry monitors this increase in cur-
rent (the threshold is set to approximately 10 Amps) and shuts
off the power device as quickly as possible in the event of an
overload condition. In a typical motor driving application the
most common overload faults are caused by shorted motor
windings and locked rotors. Under these conditions the in-
ductance of the motor (as well as any series inductance in the
V_CC supply line) serves to reduce the magnitude of a current
surge to a safe level for the LMD18200. Once the device is
shut down, the control circuitry will periodically try to turn the
power device back on. This feature allows the immediate re-
turn to normal operation in the event that the fault condition
has been removed. While the fault remains however, the de-
vice will cycle in and out of thermal shutdown. This can create
voltage transients on the V_CC supply line and therefore proper
supply bypassing techniques are required.
The most severe condition for any power device is a direct,
hard-wired (“screwdriver”) long term short from an output to
ground. This condition can generate a surge of current
through the power device on the order of 15 Amps and require
the die and package to dissipate up to 500 Watts of power for
the short time required for the protection circuitry to shut off
the power device. This energy can be destructive, particularly
at higher operating voltages (>30V) so some precautions are
in order. Proper heat sink design is essential and it is normally
necessary to heat sink the V_CC supply pin (pin 6) with 1 square
inch of copper on the PCB.
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