This column is about the absolute maximum ratings of motor drivers. Although the title has the phrase "motor drivers", the definition of absolute maximum ratings is not limited to motor drivers, but is the same even when the scope is extended to other semiconductor devices. This column addresses the subject in the interest of fostering a proper understanding of the definitions and intentions of absolute maximum ratings, so as to avoid related problems.
About Absolute Maximum Ratings
For caution's sake, we begin by reviewing the definition of the term "absolute maximum rating". With respect to semiconductor devices, the definition essentially relies on the term in "JIS C 7032 General rules for transistors", the definition of which is "limit values, which must not be exceeded even momentarily, and any two of which must not be reached simultaneously". Put simply, this is interpreted to mean " the value of any item in the absolute maximum ratings must never be exceeded, not even for a very short time".
Absolute Maximum Ratings of Motor Drivers
In motor driver design, impulse-shape current, that is, current spikes lasting short times, flow in the output current, and we are asked if there is no problem when the current value momentarily exceeds the absolute maximum rating. The answer here is clear from the above definition of absolute maximum rating: such events cannot be allowed. An absolute maximum rating does not allow for discussions of tolerances or momentary occurrences (though there are specification items for which a time is stipulated).
Let us look at an example of an actual absolute maximum rating for a motor driver IC. The following is an excerpt from the data sheet for a ROHM H bridge circuit-incorporated type driver IC for a brushed DC motor.
In this example, the absolute maximum rating for the output current is 3.0 A, and so in essence, this value must not be exceeded; however, below this is the item "output current (peak)", and considering this together with Note 2, for a pulse with a pulse width of 1 ms or shorter and a duty cycle of 20%, the absolute maximum rating is 4 A. If the above-described impulse is within these limits it is allowed, but in any case, exceeding the absolute maximum rating is not acceptable. It should also be noted that in this example, there are specification items for the output current that have time constraints, but these are not necessarily present in all motor driver ICs. Items with absolute maximum ratings also depend on the manufacturer and model, and are not uniform.
Further, The absolute maximum rating of 3.0A for output current is accompanied by Note 1: " Do not exceed Tjmax = 150°C". This means that, from the standpoint of output current alone, up to 3.0 A can be allowed, but the current actually flowing must be such that Tjmax does not exceed 150°C. In the definition of absolute maximum rating, none of the items for which there is a stipulation must be exceeded; but in cases where there is such an interrelation between current and heat generation, there are cases in which, even when an absolute maximum rating is not reached, a limit is imposed by another parameter.
Cases Where an Absolute Maximum Rating Has Been Exceeded
When an absolute maximum rating is exceeded, in general there is the possibility of degradation of characteristics and the voltage rating and the possibility of failure; and even if no abnormality is observed immediately afterward, the operation lifetime may be shortened. There are specification items for which immediate failure results when the absolute maximum rating is exceeded even slightly, but there are also items for which this is not the case. However, the latter does not mean that "it was just for a moment" is an excuse. We occasionally hear people say that "it's dangerous to ignore current limits, but some temperature overruns are okay", but not only is there no basis for this, it is a representative case of misunderstanding the definition and meaning of absolute maximum ratings.
Hence when in the design process there are circumstances in which an absolute maximum rating is exceeded, we must choose a design that keeps within the absolute maximum rating, or else devise a countermeasure, or study the possibility of using devices and components with absolute maximum ratings for which the circumstances can be allowed.
The Relation of Absolute Maximum Ratings to Recommended Operating Conditions and to Specifications
In addition to absolute maximum ratings, a data sheet also presents "recommended operating conditions" and "specifications". Where a proper understanding of absolute maximum ratings is concerned, after the definition itself, there is another important matter: the fact that "even when absolute maximum ratings are given, they are the values that can be allowed, but they are not values for which operation and characteristic values are guaranteed".
The latter values are in fact the "recommended operating conditions", which are also shown on the same data sheets for motor driver ICs.
Where the output current (continuous) is concerned, the recommended operating condition is a maximum 2.4 A, which is smaller than the absolute maximum rating of 3.0 A. This is the condition for using the device so as to ensure the specified characteristic values and the expected reliability.
In an example of a power supply voltage, whereas the absolute maximum ratings are -0.2 V to 36.0 V, the recommended operating conditions are 8 V to 28 V. This example may be easier to understand. The minimum value of 8 V indicates that if the power supply voltage for this motor driver IC is below 8 V, the IC may not operate properly. Of course, there is no problem with applying 5 V, 3.3 V, and so on as the power supply, but the device probably will not operate. It should also be noted that as the power supply voltage condition in the specifications for electrical characteristics, 24 V, which is the typical value for recommended operating conditions, is a basic requirement.
To sum up, an absolute maximum rating is a simple yet rigorous value that must never be exceeded. This is not limited to motor driver ICs, but is common to all semiconductor devices. During both design and evaluation, actual operation under a variety of assumed conditions must be checked, and of course it must be confirmed that absolute maximum ratings are not exceeded, but as a general rule, it is necessary to confirm that operation accords with the recommended operating conditions. In particular, pulse-shape voltages and currents must be checked.