AN-2083: Integrated Power Supply Solution for Vehicle Tracking System
INTRODUCTION
A vehicle tracking system (VTS) is typically installed in cars and trucks. A VTS provides real-time information about the speed, location, and direction of the vehicle using technology, such as a global positioning system (GPS).
For a VTS to work reliably, it needs to have a robust dc-to-dc power supply. A power supply must be designed to protect the system from common automotive fault conditions like load dump, cold crank, reverse polarity, and other potential destructive transients described in the ISO 7637-2 and ISO 16750-2 standards.
In addition, in the absence of power coming from the vehicle, the system must be able to switch to the backup battery to ensure continuous operation. Ultimately, an automotive power supply must comply with automotive electromagnetic interference (EMI) standards, specifically CISPR 25.
To address these challenges, Analog Devices, Inc., developed the EVAL-ADVTS4152-EBZ power supply solution, which is a high performance, simplified power supply solution for vehicle tracking systems.
INTEGRATION SOLUTION
The EVAL-ADVTS4152-EBZ power supply solution integrates three main blocks: the LT4356-1 surge stopper, the efficient LT8609A step-down regulator, and the LTC4040 battery backup power manager. These three blocks work together to provide reliable and efficient power to downstream electronics like a VTS.
The EVAL-ADVTS4152-EBZ is compatible with a nominal 12 V or 24 V system and designed to output 5 V and deliver 3 A of continuous current. When the power from a car battery is not available, the system automatically switches to a backup supply from a single cell Lithium Ion (Li-Ion) or Lithium Iron Phosphate (LiFePO4) battery.
| Parameter | Test Conditions/Comments | Min | Typ | Max | Unit |
| INPUT Operating Range Positive Surge Protection Reverse Protection |
Suited for 12 V and 24 V power system Main Battery |
6.5 2.7 -40 |
12 or 24 |
38 5 200 |
V V V V |
| OUTPUT Voltage Ripple Current |
Output = 5 V, 2 A Main 12 V (buck mode) Battery 3.6 V (boost mode) |
4.5 |
3.842 9.522 |
5.5 3 |
V mV mV A |
| BACKUP BATTERY Operating Range Selectable Charging Voltage |
Li-Ion LiFePO4 |
2.7 |
3.95, 4.0, 4.05, or 4.1 3.45, 3.5, 3.55, or 3.6 |
5 |
V V V |
SURGE STOPPER
Designing an automotive power supply to handle high voltage transients can be challenging because this requires a device to dissipate excess power while leaving sensitive electronics undamaged. The EVAL-ADVTS4152-EBZ power supply solution features the LT4356-1 surge stopper that protects the system from high voltage transients and continues to operate during events like these.
The LT4356-1 drives an external N-channel, metal-oxide semiconductor field effect transistor (MOSFET) as a pass transistor (see Figure 3). In normal operation, the LT4356-1 fully turns on the MOSFET and provides a low impedance path from the input to the downstream load. If the input voltage surges too high, the LT4356-1 controls the gate of the MOSFET and regulates the output to a safe voltage level set by the R33 and R34 resistive divider from the OUT pin to ground. The MOSFET stays on until the capacitor charge (CTMR) connected at the TMR pin to ground reaches 1.35 V. At that point, the GATE pin pulls low turning the MOSFET off.
Load Dump
A load dump is an example of the high voltage transients. A load dump happens when the alternator disconnects from a car battery while being charged causing a voltage surge. The EVAL-ADVTS4152-EBZ uses an LT4356-1 surge stopper to protect sensitive automotive electronics from this kind of transient condition.
Test Setup
A load dump test pulse was generated using a DC1950A load dump generator and sent across the inputs of the EVAL-ADVTS4152-EBZ. A 1 A constant current load is then applied across the output of the EVAL-ADVTS4152-EBZ. Figure 4 shows the test setup for a load dump condition.
The actual surge waveform, shown in Figure 5, displays in Channel 2 (C2) and reads a 105.6 V surge that decays around 400 ms. The voltage reading across the output is kept at 5 V and is shown in Channel 3 (C3), indicating that the power was not interrupted. In Figure 5, the LT4356-1 response is also shown in Channel 1 (C1). The flat region in the waveform shows that the chip is regulating the voltage at the desired 38 V clamp voltage (see Figure 5).
STEP-DOWN REGULATOR
The output of the LT4356-1 is then connected to the input of the LT8609A high efficiency, step-down regulator (see Figure 6). The LT8609A has a wide input range from 3.0 V to 42 V. A resistor tied between RT and ground is used to set the switching frequency. A SYNC pin allows users to enable the spread spectrum modulation for low EMI operation.
BATTERY BACKUP POWER MANAGER
For the VTS to function continuously, it must have a backup power supply in the absence of power from a car battery. The EVAL-ADVTS4152-EBZ has a LTC4040 high current, step-up, dc-to-dc regulator that backs up the supply from a single cell, Li-Ion or LiFePO4 battery (see Figure 7).
When the output of the LT4356-1 drops below the 1.2 V power fail input (PFI) threshold, the 2.5 A boost regulator delivers power from the backup battery to the downstream load.
Normal to Backup Battery Mode
To simulate brownout conditions, the power supply across the input of the EVAL-ADVTS4152-EBZ was disconnected to see the transitioning waveform from normal operation to backup battery mode. Figure 9 shows the test setup. This simulation also shows how the EVAL-ADVTS4152-EBZ continues to operate during undervoltage transients such as cold crank.
In Figure 8, a nominal 12 V input is displayed in Channel 1 (C1). The system output displayed in Channel 3 (C3) shows that the 5 V output was uninterrupted as the 12 V input drops. This result also shows that the EVAL-ADVTS4152-EBZ can continuously operate even during cold crank transients.
Choosing A Charge Voltage for the Battery
When car battery power is available, the step-up regulator operates in reverse as a step-down battery charger. The charging voltage can be configured according to the battery type via a user-selectable pin. The LTC4040 offers four different charge voltage options for each of the two battery chemistries (Li-Ion and LiFePO4), and these options are selected through the S1, S2, and S3 slide switch inputs. Table 2 shows the switch configurations for choosing a charge voltage for the two types of batteries.
| Battery Type | Switch Setting | Min | Typ | Max | Unit |
| BATTERY REGULATED OUTPUT VOLTAGE For LiFePO4 Option For Li-Ion Option |
S1 = 0, S2 = 0, S3 = 0 S1 = 0, S2 = 1, S3 = 0 S1 = 0, S2 = 0, S3 = 1 S1 = 0, S2 = 1, S3 = 1 S1 = 1, S2 = 0, S3 = 0 S1 = 1, S2 = 1, S3 = 0 S1 = 1, S2 = 0, S3 = 1 S1 = 1, S2 = 1, S3 = 1 |
3.42 3.47 3.52 3.57 3.92 3.97 4.02 4.07 |
3.45 3.50 3.55 3.60 3.95 4.00 4.05 4.10 |
3.48 3.53 3.58 3.63 3.98 4.03 4.08 4.13 |
V V V V V V V V |
THERMAL SHUTDOWN PROVISION
The EVAL-ADVTS4152-EBZ has an optional ADT6401 pin-selectable temperature switch for over temperature protection if the design needs to protect from abnormally high dc voltage. For reference, see the EVAL-ADVTS4152-EBZ, UG-1916 for the ADT6401 schematic.
PRECOMPLIANCE TESTING
ISO 7637-2:2011 AND ISO 16750-2:2012 STANDARDS
The ISO 7637-2:2011 and ISO 16750-2:2012 standards describe the possible transients and specify the test methods to simulate the transients. The test requirements required to comply are shown in Figure 10 as a reference.
CISPR 25 RADIATED AND CONDUCTED EMISSION
CISPR 25 is an automotive standard that regulates radiated and conducted emissions for the protection of on-board receivers in a vehicle. The radiated EMI performance of EVAL-ADVTS4152-EBZ is shown in Figure 11 and Figure 12. While the conducted EMI performance is shown in Figure 13 and Figure 14. The red lines in Figure 11 through Figure 14 represent the CISPR25 Class 5 radiated and conducted emission peak limits.
Authors
