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Back-up Battery Power Reference Design for Automotive Emergency Calling Applications


This is really a backup power reference design for automotive E-Call application. It uses one cell LiFePO4 as the backup battery. The minimum backup battery voltage can be down to 2 V with boost converter TPS61088-Q1. The boost converter TPS61088-Q1 can output 8V/1.6A at 2V input voltage. The value it has over the integrated solution levels of competition are far better efficiency and less redundancy when boosting in the back-up battery.


  • Wide input voltage range: 3.8 – 36 V
  • 8 V/1.6A output power capability when backup battery voltage drop to two V
  • Longer Battery Lifetime
  • Low cost
  • Simple and flexible


Emergency call (eCall)

Telematics control unit

1 System Description

In the E-call application, once the main battery voltage is high, all of the related circuits in the E-call is supplied through the main battery via a buck converter. Once the main battery voltage drops to some low-level, the buck converter browns out, the back-up battery starts working, it increases the equipment via a boost converter. Figure 1 shows a traditional E-Call block diagram. Besides a high-voltage buck converter,

Figure 1. Traditional E-call Block Diagram

The TIDA-050031 takes into account cost and the size while avoiding the redundancy of the traditional e-Call block diagram in Figure 1. This reference design offers a simple and inexpensive backup power circuit for the automotive E-Call application. The dwelling really is easy. Just one boost converter is used in the circuit. The boost converter TPS61088-Q1 can output 8V/1.6A rich in efficiency at 2V input voltage. So the minimum backup battery voltage can be right down to 2V. Besides extending the lifetime of the backup battery, the TIDA-050031 is also flexible and less redundancy.

1.1 Key System Specifications

Table 1 offers the TIDA-050031 performance specification. The linear charger charges the back-up battery with a constant 100mA current. The maximum backup power can reach 12.8W once the backup battery drops right down to 2 V.

Table 1. Key System Specifications

Linear Charger Current 100mA ± 25% tolerance
Maximum Backup power 8 V/1.6A @2 V backup battery voltage


2 System Overview

2.1 Block Diagram

Figure 2 shows the block diagram of TIDA-050031. The output voltage from the buck converter is set at 9 V. The backup battery is charged with this 9 V voltage with the linear charger. After the main battery voltage drops low and the buck converter LMR33630-Q1 browns out, the linear charger reduces. The backup battery increases the energy for the E-Call system with the boost converter TPS61088-Q1.

Figure 2. TIDA-050031 Block Diagram

2.2 Design Considerations

A key feature of the reference guide is the fact that once the buck converter browns out, the TPS61088-Q1 boost converter provides voltage to the system immediately. The TPS61088-Q1 boost converter is always enabled to work. Once the buck converter works normally and output 9 V, this 9 V voltage added in the creation of the TPS61088-Q1, helps make the FB pin voltage greater than 1.3V, so the TPS61088-Q1 stops switching, it really works in the low power dissipation mode, the present draws from the input side is under 3 uA.

2.3 Highlighted Products

This reference design adopts the next devices:

  • TPS61088-Q1 is a 10-A Fully-Integrated Synchronous Boost Converter
  • LMR33630-Q1 is really a wide input voltage 3-A Synchronous step-down voltage converter
  • TPS7A2501 is a 300-mA, 18-V, Ultra-Low IQ, Low-Dropout Linear Voltage Regulator with Power-Good
  • ATL431AIBDZR is a Low Iq Adjustable Precision Shunt Regulator

2.3.1 TPS61088-Q1

The TPS61088-Q1 is a 2.7 V to 12 Vin high power density, synchronous boost converter made to provide a high quality, small size solution for automotive applications. The TPS61088-Q1 includes a min input voltage of 2.7 V and for that reason can act as a boost for a single or two cell Li-Ion Back-Up Battery (BUB) in applications for example E-Call that also require a high output they are driving the speaker, antenna along with other circuits.

2.3.2 LMR33630-Q1

The LMR33630-Q1 automotive-qualified regulator is definitely an easy-to-use, synchronous, step-down DC/DC converter that delivers best-in-class efficiency for rugged applications. The LMR33630-Q1 drives up to 3 A of load current from an input of up to 36 V. The LMR33630-Q1 provides high light load efficiency and output accuracy in an exceedingly small solution size. Features like a power-good flag and precision enable provide both flexible and easy-to-use solutions.

2.3.3 TPS7A2501

The TPS7A2501 low-dropout (LDO) linear voltage regulator introduces a mix of a couple.4-V to 18- V input voltage range with very-low quiescent current (IQ). These features help modern appliances meet increasingly stringent energy requirements, and help extend battery life in portable-power solutions. The TPS7A25 is available in both fixed and adjustable versions. For additional flexibility or higher output voltages, the adjustable version uses feedback resistors to create the output voltage from 1.24 V to 17.64 V. Both versions possess a 1% output regulation accuracy that provides precision regulation for most microcontroller (MCU) references.

2.3.4 ATL431AIBDZR

The ATL431AIBDZ are three-terminal adjustable shunt regulators, with specified thermal stability over applicable automotive, commercial, and industrial conditions. The output voltage could be set to any value between Vref (approximately 2.5 V) and 36 V, with two external resistors.

2.4 System Design Theory

Figure 3 shows the linear charger circuit of TIDA-050031. The benefits of the linear charger are simplicity and inexpensive. Transistors Q2 and Q3 are put into limit the initial charge current and makes the charging current a continuing value. The existance of Q2 and Q3 may also avoid Q1 being damaged inside a short-circuit condition.

Figure 3. Linear Charger Circuit

Figure 4 shows the TPS61088-Q1 boost converter of TIDA-050031, which is the main circuit of TIDA050031. The Vin pin of TPS61088-Q1 is supplied through the output voltage through a diode. Once the main battery can be obtained, the buck converter is working, Vout is around 8.7V, therefore the Vin pin voltage is around 8.4V considering the forward voltage drop of D2. The EN pin is pulled high to Vcc. So once the main battery is available and the buck converter starts working, the TPS61088-Q1 is enabled at the same time. Within this condition, the operating quiescent current at the Vin pin is less then 3uA, which quiescent is sucked from the Vout rather than the backup battery. So the backup battery lifetime could be greatly prolonged in this configuration.

Figure 4. TPS61088-Q1 Boost Converter

Figure 5 shows the operation sequence of TIDA-050031. Sometimes t0, main battery ready. Buck converter starts working. At time t1, LDO output 5V, the PG pin of the LDO gets high, linear charger begin working, it charges the backup battery having a 100mA constant current. Sometimes t2, the primary battery browns out, linear charger stops working, the E-Call product is run by the backup battery although the TPS61088Q1 boost converter. At time t3, the primary battery recovery, it give you the E-call system with the buck converter again.

Figure 5. Operation Sequence

3 Hardware, Software, Testing Requirements, and Test Results

3.1 Required Hardware and Software

3.1.1 Hardware

This reference design uses the next hardware to complete the measurement:

  • 10A output capability DC power supply
  • 2A rating E-load
  • Digital oscilloscope
  • Signal generator

3.2 Testing and Results

3.2.1 Test Setup

This section describes how to properly connect and hang up the TIDA-050031:

  1. Enable the Buck converter, LDO and the TPS61088-Q1 boost converter;
  2. Connect the positive terminal of DC power supply to J1(Vin), its GND terminal to J2 ( GND);
  3. Turn on the DC power, set the load current to 1.5 A;
  4. After one hour, when the backup battery charges up, turn off the DC power;
  5. TPS61088-Q1 starts working;
  6. Turn from the DC power.

3.2.2 Test Results

Figure 6 and Figure 7 show the charge voltage and also the charge current across a 2000mAh LiFePO4 capacitor. The initial charge current is well limited. The charge current is 100 mA.

Figure 6. Charge Voltage and Charge Current-1

Figure 7. Charge Voltage and Charge Current-2

Figure 8 shows the conversion efficiency of the TPS61088-Q1 boost converter under 1.5A load current.

The conversion efficiency is very high even the backup battery voltage drops to two V.

VIN (V) D001


About the Author

Helen Chen has more than 15 years of application experience of power-supply product design. She's acquainted with various topologies such as RCC, buck-boost, full-bridge, half-bridge, flyback, CCM PFC, and DCMB PFC. She is also familiar with the magnetic components design, PCB layout, and EMI solutions.