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 maximum 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.

Therefore, 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

transistor. 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 ser-

vice routine would then be designed to take appropriate

steps, such as reducing load currents or initiating an orderly

system shutdown. 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 tran-

sients 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

(transorb) 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

driving high load currents a greater amount of supply bypass

capacitance (in general at least 100 µF per Amp of load

current) 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

important to consider the effects of the substantial surge

currents through the device that may occur as a result of

shorted loads. The protection circuitry monitors this increase

in current (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 con-

ditions the inductance of the motor (as well as any series

inductance in the V

magnitude of a current surge to a safe level for the

LMD18200. Once the device is shut down, the control cir-

cuitry will periodically try to turn the power device back on.

This feature allows the immediate return to normal operation

in the event that the fault condition has been removed. While

the fault remains however, the device will cycle in and out of

thermal shutdown. This can create voltage transients on the

V

niques 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

01056824

FIGURE 4. Transitions in Brake, Direction, or PWM Must Be Separated By At Least 1 µsec

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