Question: Although there is a provision of ○○A included in the absolute maximum rating for output current, current flow under impulse conditions may exceed this value. Is this current allowable for a short (instantaneous) period of time?
Answer: Unfortunately, there are no (time) provisions that can increase the instantaneous current value of the peak current or other parameter, and in the case that there is a maximum rating only for output current, this value must never be exceeded – even momentarily. Therefore, it is necessary to either ensure that instantaneous current does not occur or use products with a larger maximum rated value.
The absolute maximum ratings listed in the specifications indicate parameters that must never be exceeded, even momentarily. Excessive current flow (both in/out) may lead to characteristics degradation and/or damage.
In addition, IC reliability and quality can vary significantly depending on usage conditions. Even with products with the same quality and reliability, utilizing under harsh environments may reduce reliability, and conversely operating under milder conditions can increase reliability. In addition, usage under harsh conditions similar to lifetime testing, even if within the maximum ratings, can lead to wear-out failure.
The output current is the maximum current that can flow through the output and power supply pins at the power stage as well as the ground pin without degrading or damaging the internal element or bonding wires. In particular, if not specifically mentioned, current flow will be continuous. Therefore, suppliers must stipulate the maximum permissible package loss and ASO (Area of Safe Operation) range that must not be exceeded.
A typical graph of ASO（Area of Safe Operation, also referred to as SOA – Safe Operation Area）is shown below.
Figure 1. ASO Graph
Even within the rated current ④ and voltage ① limits, the SOA is limited by power dissipation ③, and damage due to secondary breakdown of the element ②. In power MOSFETs as well there is a region similar to ② above that limits the ASO caused by excessive heat concentration in a specific area. Because of this – and especially when using an inductance load – transient voltages and currents will be present due to phase differences, making it necessary to ensure that the ASO region is never exceeded. This requires waveform monitoring of transient phenomena to determine whether the voltage and current are in phase and compare with the ASO data of each transistor in individual ICs.
Over Current Protection and Output Current
Overcurrent protection circuits prevent damage caused by overcurrent conditions resulting from ground faults, load shorts, and the like. Protection operation is initiated once the set current exceeds a specified period of time, turning off all normal transistors and making the impedance high. Typically the set current is higher than the absolute maximum rating. The purpose of this is to prevent damage to the IC itself during abnormal conditions and is not designed to protect the set. For this reason it is necessary to ensure use so that the maximum rated output current is never exceeded, since the instantaneous current cannot be suppressed by using the overcurrent protection function within the absolute maximum rated value.