An automotive sensor supply can be built without the conventional switching pre-regulator if the linear pass stage is only enabled when the sensor output actually has to be sampled. The circuit targets regulated outputs of 5 V or 3.3 V, depending on the CPUcentral processing unitMain processor that executes program instructions and performs data operations. logic bus, and uses a transistor linear regulator with an integrated short-circuit trip instead of a DC-DCdirect current to direct currentA converter that changes one DC voltage level to another. converter feeding a pre-regulated 6.5 V rail.
The main reason for this approach is power dissipation. A simple LDOlow-dropout regulatorA linear regulator with a small input-to-output voltage difference. would otherwise burn significant power when driven directly from the vehicle battery, even with only a few tens of milliamps of sensor load. By switching the series pass transistor off for long periods, the design avoids continuous dissipation and makes the pre-regulator unnecessary in many sensor applications.
The circuit uses Q1, an MPSA42 NPN1. negative-positive-negative2. n-type, p-type, n-type bipolar transistorThis transistor polarity is used for Q1, Q2, and Q3. series pass transistor, together with Q2 and Q3 as current-limit and control stages, and Q4, a MPSA92 PNPp-type, n-type, p-type bipolar transistorp-type, n-type, p-type bipolar transistor transistor that isolates the base drive to Q1. The current limit is set by R4 at about 70 mA, which sits just above the stated 50 mA output rating. A 6.2 V Zener diode with D3 creates a reference of roughly 7 V, and the downstream transistor and diode drops produce a nominal regulated output of around 5 V.
The CPUcentral processing unitMain processor that executes program instructions and performs data operations. pin has two functions. It senses the sensor supply through the R1/R2 divider, which scales the voltage to about 2.5 V for analog measurement, and it can also disable the supply by forcing the control node low. A low level turns off Q3, then Q4, and finally removes the base drive from Q1. Diode D4 helps keep Q3 off even if the CPUcentral processing unitMain processor that executes program instructions and performs data operations. output low level is not ideal.
This operating mode assumes the sensor is only read for a few microseconds every 100 milliseconds, and for slowly varying signals such as temperature perhaps only once per second. In such cases, sensor acquisition is unnecessary for more than 99% of the total time, so the output stage can remain disabled most of the time without affecting the measurement task.
The design also includes automotive protection against a short to battery via the Schottky diode D1. For higher bus voltages such as 24 V or 48 V, the resistor values and transistor ratings must be adapted, and a transistor with stronger thermal capability may be needed if the pass device is enabled only for brief intervals.