Different protection for different conditions
A problem in battery-powered systems is the threat of damage to the IC and/or the load when an end-user inserts the battery with reverse polarity or misconnects the battery, i.e. in a vehicular system. If an IC is exposed to a reverse voltage, large currents will flow to ground through parasitic junctions in the IC, potentially destroying fragile junctions. Insertions of single or multiple diodes help, but these fixes waste power and reduce the supply voltage by adding diode drops between the battery voltage and the supply rail. An alternative on-chip solution would not only protect the IC and the load but eliminate those tradeoffs resulting from adding more components.
Reverse output voltage protection prevents reverse current from flowing through the IC’s parasitic body diodes under a reverse output voltage condition, or if the load is returned to the negative supply rail, or if the negative supply is turned on before VIN, or if the output is sitting at the negative rail’s potential during power up.
Linear regulators can be easily destroyed if they are forced to source excessive current. Therefore, current limiting (short circuit) protection circuitry kicks under short circuit or excessive load conditions in which VOUT<VIN; the current limiting circuitry prevents excessive current flow from VIN to VOUT. In a short circuit condition, not only is the pass transistor sourcing excessive current, the voltage across it is at a maximum (since VOUT is at ground, the voltage across the transistor is VIN.). Linear regulators typically use one of two types of short circuit protection on-chip: constant current limit or a more sophisticated form, current limit with foldback. The addition of foldback protection to the current limit decreases the current limit as the input voltage increases to keep the power transistor in its safe operating area (SOA).
When VOUT>VIN, i.e. if VIN is shorted or VOUT is pulled above VIN, the reverse current (a.k.a. reverse output-to-input) circuitry prevents any flow of reverse current from VOUT to VIN.
With thermal shutdown protection, an IC is actually shut off and the die must cool down by the amount of hysteresis built into the thermal shutdown circuitry. Once the part has cooled down sufficiently, it is restarted. If a fault or overload condition exists, the part heats back up to the thermal shutdown temperature and turns itself back off. Therefore, the part sits and thermally oscillates at some low frequency and duty cycle depending on the thermal shutdown temperature, the amount of hysteresis, the package and the associated thermal time constants.
Thermal limiting is a slightly less sophisticated technique than thermal shutdown, in which the maximum die temperature is controlled by the protection circuit.
The ideal solution to the problems and required protection features outlined above would be a family of low dropout linear regulators that can operate with high precision and offer bulletproof protection features across the entire junction temperature range of -55°C to +125°C.
Linear Technology has provided bulletproof, reliable, high performance PNP and NPN LDO regulators operating across the extended industrial junction temperature range of -40°C to +125°C for many years. However, sophisticated wafer fabrication technology enhancements and improved layout techniques have allowed extension of the operating junction temperature range down to -55°C, opening up a variety of “high reliability” military range applications, unleashing a new generation of military plastic or “MP-grade” devices.
Linear’s new MP-grade portfolio encompasses output currents from 20mA (LT3008) all the way up to 5A (LT3070), with dropout voltages as low as 85mV, input voltages as high as 80V, ultralow output voltage noise as low as 20uVrms, sub-5uA quiescent currents and a full set of protection features. See Table 1
for a portfolio summary.
Table 1: Linear Technology’s family of MP-grade single output LDOs
(Click on image to enlarge)