eTracker Reference Design

Automotive Tracker with Linear Charger


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1.1 Description

Tracking the location of different kind of vehicles is aimed to be a widely used application for many purposes, such as security. However, this presents two challenges in electronic design as the GSM modules, which establishes the communication with the receiver station, have strict input operating voltage ranges and, in addition, the system must keep tracking the location when the car battery is turned off.

The first challenge of this design is to maintain the communication when the car battery is disconnected. The presented solution consists in adding an external Li-Ion Battery Pack that supplies the required power to the GSM module when the car battery is turned off. Moreover, in order to maintain the charge of the battery, a battery charger is connected to the input stage. Then, when car battery supplies 12V, the battery charger and the GSM module are powered from it and, therefore, the external battery enter in charging mode. On the other side, when the car battery is disconnected, the external battery pack supplies the power to the GSM module.

The second challenge consists in adjusting the input voltages of the battery charger and the GSM module taking into account their specifications. Commonly, the GSM modules have an input operating voltage range that varies from 3.4V to 4.2V, while the battery charger range varies from 4.05V to 6.05V. Then, a buck converter is required in order to decrease the input voltage from 12V (car battery) to 4.2V. Moreover, is required also to protect the GSM module from load transients. For this reason, a diode is added to decrease the voltage at the output of the buck below 4V.

This reference design will help engineers designing a simple power stage for a common tracker.

1.2 Features

  • Wide Operating Input Range (from 4.2V to 36V)
  • 80mA Continuous Output Current
  • 350kHz to 2.5MHz Programmable Buck Switching Frequency
  • Second-Order EMI Filter
  • Reverse Polarity Protection
  • Low Dropout Mode
  • Fully-Autonomous Charger
  • Programmable Charge Current: 30mA to 1A

1.3 Applications

  • Car Tracking
  • Internet Connectivity
  • 2G Communications (Calls, SMS and MMS)
  • Telematic services

Figure 1: MPS eTracker Reference Desgin Board

Reference Design

2.1 Block Diagram

Figure 2: Block Diagram

The circuit contains 5 main blocks. First, a buck converter is used to reduce the supply voltage from the 12V of the car battery to a stable 4.2V. The design limits the input voltage to 36V (due to the selected Step-Down Converter specifications), although, the design allows to change the converter to reach higher voltages without implying big changes to the design. Then, a Li-Ion battery charger is added to charge the external battery pack. Thirdly, the Vin detection circuit detects the state of the car battery to activate the Li-Ion Battery Pack supply. Finally, the EMI Filter and the Reverse Battery Protection is used to protect the circuit from non-ideal behaviors.

2.2 Related Solutions

This reference design is based on the following MPS solutions:

MPS Integrated Circuit Description
MPQ4433 Synchronous buck converter with a wide input voltage range and up to 3A of output current
MP26029 Li-Ion/Li-polymer battery charger IC with thermal regulation

Table 1: System Specifications

2.3 System Specifications

Parameter Specification
Input voltage range 4.2V to 36V
Output voltage range 3.3V to 4.1V
Nominal load 4V / 80mA 
Maximum Peak Output Current1 3000mA
Switching frequency 450kHz (under nominal conditions)
Board form factor 100mmx100mmx2mm 
Converter efficiency (Vin = 12V) 94%
4V output ripple 31.25 mV
Load transient (2A - 80mA) 270 mV
Charge current range 30mA to 1A (set to 110mA)
Battery voltage range 3.6V to 4.2V (set to 4.2V)
Vin Quiescent Current (ILOAD = 0A) 700uA
Li-Ion Battery Quiescent Current 45uA
Shutdown Current (MPQ4422 OFF) 2uA

Table 2: System Specifications

Test Results

3.1 Efficiency and Regulation

VOUT = 4.2V, L = 8.2µH, fSW= 454kHz, TA = 25°C.


Figure 10: Efficiency vs. Load Current - Buck Converter


Figure 11: Efficiency vs. Load Current - System without battery

3.2 Time Domain Waveform

VIN = 12V, VOUT = 4.2V, L = 8.2µH, fSW= 454kHz, TA = 25°C.

3.2 1

3.2 2

3.3 EMC Measurement

150kHz to 108MHz


Figure 33: CISPR25 Class 5 Conducted Emissions

150kHz to 30MHz


Figure 34: CISPR25 Class 5 Radiated Emissions