An automotive sensor power supply appears deceptively simple to design but in fact poses many formidable challenges. The regulated output voltage is required to be 5V or 3.3V, depending on the logic bus voltage for the CPU. A simple low-dropout (LDO) regulator may dissipate considerable power across it even with a few tens of mA total sensor load.
The traditional approach uses a DC-DC convertor that provides a pre-regulated supply bus of around 6.5V. Note, however, that the output also needs to be protected from short-circuits to both ground and the battery bus. Hence a conventional sensor supply solution uses a three-terminal regulator IC-based power supply, preceded by a switching pre-regulator.
Wow the engineering world with your unique design: Design Ideas Submission Guide
The circuit showcased in this Design Idea (Figure 1) gets rid of the DC-DC converter and also incorporates an in-built low-cost short-circuit trip based on a transistor linear regulator.
Figure 1 Component identities and values for the circuit showcased in this Design Idea are included in the list that follows.
- Q1, Q2 and Q3 = NPN MPSA 42
- Q4 = PNP MPSA 92
- R1,R2 = 1 KΩ
- R3 = 10KΩ
- R4 = 10 Ω
- R5 = 47 KΩ
- R6 = 100 KΩ
- R7 = 1 KΩ
- R8 = 100 Ω
- D3,D4 = 1N 914
- D2 = Zener 6.2V, 500mW
- D1 = Shottkey diode RB521 CS 30 L
- C1 = 100 KF, 50V
A single CPU pin interfaces with the sensor supply output circuit as shown. The CPU pin plays a dual role:
- It senses the sensor supply output through the R1,R3 potential divider. This is required for scaling the sensor outputs to ensure measurement accuracy. The CPU pin needs to be configured as an analog input for this purpose.
- It can be used to disable the sensor supply output by turning off the series pass transistor Q1. The CPU pin needs to be configured as an output for this purpose. A logic low level issued at the CPU output pin turns off Q3. The diode D4 ensures holding Q3 off even if the worst-case low-level CPU output is around 0.7V.
The series pass transistor can be turned off to conserve the power being dissipated in it when analog input sensing is not required. This system design countermeasure allows us to omit a switching pre-regulator to conserve continuous power dissipation across the series pass transistor.
Careful analysis of the sensor signal acquisition task reveals that the sensor signal typically needs to be read only for a few microseconds every, say, 100 milliseconds. Some slowly varying signals such as temperature may even be read only every second. Hence, for more than 99% of the total time, sensor signal acquisition is not necessary. We can therefore safely disable the sensor supply by turning off its series pass transistor.
As discussed previously, the series pass LDO is responsible for power dissipation if it is directly powered from the vehicle battery, since it drops a large voltage across it. But if we cleverly keep it off most of the time, we do not really need any DC-DC pre-regulation to conserve power.
Analysis of various current levels and role/values/ratings of various components are as follows:
- Q1: MPSA42 is a 500 mA NPN transistor with a minimum current gain of 40. A suitable higher-rated transistor may be used for higher current applications.
- Q2: This current limit transistor with current limiting resistor R4 = 10 Ω sets the current limit to approximately 700 mV/10 = 70 mA. This is just above the rated output current of 50 mA.
- D2 is a 6.2V 500 mW Zener (1N4735) in series with D3 (1N914), together generating a voltage reference of around 7V. The downstream voltage drops of the Q1 and Q2 base emitter junctions, coupled with the Schottkey diode RB521CS30L, generates a nominal regulated voltage of around 5V.
- The R1 and R2 chain of 1 KΩ each feeds around 2.5V to the CPU monitoring the analog voltage. This CPU can appropriately scale the sensor voltages gathered by its analog inputs to attain the desired accuracy.
- Transistor Q4 is a MPSA92 PNP with 500 mA collector current and a minimum current gain of 25. It acts as an isolating switch to isolate the current drive fed to the base of series pass transistor Q1.
- R7 and R8 together (1.1 KΩ) provide a current of around 4.4 mA to the series pass transistor base along with Zener reference D2. Assuming that the Q1 base current requirement is 1.2 mA or less, the Zener/diode combination will be adequately supplied with more than 3 mA of current. This current will firmly bias the Zener in its breakdown region.
- Resistor R5, along with transistor Q3, serve to switch on/off the base current of Q4 to restore/isolate the base drive to the series pass transistor Q1. Diode D4 firmly holds Q3 in the off state when the CPU issues a logic low level (≤0.7V) at its output interface pin.
- The collector current of the transistor Q3 in it on state will be around 1.2 mA , taking into account the diode D3 drop.
- When the sensor supply is to be turned off, the CPU interface issues a logic low level to the junction point of R1/R2. This turns off Q3 and hence Q4. D4’s role is described earlier to firmly hold Q3 in its off state.
- An off-state Q4 further turns off the base current plus Zener current supply to the series pass Q1. This in effect switches off the sensor supply.
- The CPU interface needs to be judiciously tailored depending on the logic low level of the CPU output. Based on the maximum low logic level output voltage, a suitably high value series resistance/divider may need to be inserted.
- The Schottky diode D1 protects the sensor supply from short to battery field condition as dictated by automotive norms
This idea illustrates the architectural philosophy for the sensor supply architecture discussed. Based on the actual application constraints the component values will have to be tailored.
For high voltage battery buses such as 24 V or 48 V, the resistors’ values and transistor selections need to be suitably altered to meet power and voltage requirements. Since we are aiming to turn on the series pass for around a millisecond for every 0.1 second or higher, a power transistor with robust “thermal capacitance” may be needed for optimal thermal performance with higher voltage buses.
The discerning designer is expected to make suitable changes to the transistor ratings and resistor values, without violating the basic design philosophy, to suit diverse automotive scenarios such as a battery bus of 24V or 48 V. Depending on the application scenario, the designer may also choose to use a separate CPU output pin with an extra dedicated NPN transistor across R2 to switch Q3.
Vishwas Vaidya is a graduate of the Indian Institute of Technology in Delhi, India. Currently, he is self-employed as an engineering consultant and industry faculty member in the field of embedded systems for global automotive clients and high-repute academic institutions. Vishwas’ articles and research reports have appeared in many worldwide engineering publications.
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