Analysis of the application of LTC6802 in battery management system

1 Introduction

The normal use of batteries is an important guarantee for the safe and reliable driving of electric vehicles. Overcharging or overdischarging will cause serious damage to the battery, so it is necessary to strictly monitor each battery in the battery pack. LTC6802 is a highly integrated battery monitoring chip launched by Linear Technology. LTC6802 can monitor 12 batteries at the same time. Its peripheral circuit is simple. Its application in the battery management system greatly simplifies the system structure and effectively reduces the product cost. At the same time, its 12-bit high resolution also ensures the accuracy requirements of the system.

2 Introduction to LTC6802

2.1 Introduction

LTC6802 is a battery monitoring chip, which includes a 12-bit resolution analog-to-digital converter, a high-precision voltage reference source, a high-voltage input multiplexer and a serial interface. Each LTC6802 can measure the voltage of 12 series-connected batteries, with a maximum allowable measurement voltage of 60 volts. It can monitor the voltage of all batteries at the same time or monitor any battery in the series separately. The chip uses a unique level-shifting serial interface, and multiple LTC6802s can be directly connected in series without the need for optocouplers or isolation devices between chips.

When multiple LTC6802 chips are connected in series, they can work simultaneously, and the voltage measurement time of all series-connected batteries is within 13ms. To reduce power consumption, LTC6802 can also monitor the overvoltage and undervoltage status of each battery in real time. Each battery input terminal of the chip is internally connected with a MOS switch to discharge the overcharged battery.

2.2 LTC6802 Performance Summary

0.25% maximum total measurement error (from -40℃ ~ 85℃)

Stackable architecture enables 1000V+ systems

Delta-sigma ADC with inherent FIR filter processing circuit

1MHz serial interface with packet error checking

On-chip FET for battery discharge

Temperature sensor input

Built-in accurate 3V reference and 5V regulator

Diagnostics and troubleshooting

2.3 Pin Introduction

As shown in Figure 1, V+: the positive terminal of the device working power supply, the chip working power supply is provided by the battery, and V+ is connected to the total positive terminal of the battery pack; C12-C1: battery voltage input terminal; S12-S1: battery balancing control terminal; V-: negative power terminal, connected to the total negative terminal of the battery pack; VTEMP1, VTEMP2: temperature sensor input terminal; VREF: 3.075 voltage reference; VREG: linear voltage reference; TOS: chip position selection terminal in the series group; MMB: monitoring mode selection terminal; WDTB: watchdog output; GPIO1, GPIO2: general I/O port; VMODE: communication mode selection terminal; SCK I, SDI, SDO, CSBI: SPI interface; CSBO, SBOI, SCKO: SPI interface for communicating with the next level chip when cascading.

Figure 1 LTC6802 chip pin diagram

2.4 Working Principle

2.4.1 Delta-sigma analog-to-digital conversion

As shown in Figure 2, LTC6802 connects the input battery voltage to the 12-bit delta-sigma ADC through an input multiplexer. The internal 10ppm voltage reference source provides a reference source for high-precision ADC conversion for LTC6802. LTC6802 contains a second-order delta-sigma ADC. The ADC can eliminate the high-frequency noise generated during the conversion process by using a reconstruction filter, thereby providing a high-precision digital output, followed by a first- and second-order FIR filter. The front-end sampling frequency of the delta-sigma ADC is 512K, which greatly reduces the need for external filtering at the input end. Each conversion includes two stages, automatic zeroing and measurement.

Figure 2 LTC6802 internal structure diagram

2.4.2 Balance

LTC6802 can use both internal and external balancing methods. Each S output pin is internally connected to an N-channel MOSFET. The maximum on-resistance of the internal MOS tube is 20 ohms. When internal balancing is used, the battery is discharged through an external resistor connected in series with the internal MOSFET. The internal MOSFET can also be used to control the external balancing circuit.

In order to obtain a larger discharge current and improve discharge efficiency, external balancing is usually used. The 10K pull-up resistor inside the S pin enables its output to drive the gate of the P-channel MOSFET in the external circuit. The battery is discharged through the external series MOS tube and resistor. The on and off of the MOSFET inside the chip is controlled by the external controller to the LTC6802, and the chip itself cannot control it.

2.4.3 Open circuit detection

The LTC6802 has a unique open circuit detection function. This function ensures that the voltage reading obtained by the chip in the open circuit state will not be mistaken for a valid voltage value.

As shown in FIG3 , when there is no filtering link in the external circuit, the input resistance of AD will generate a voltage close to 0 in the open circuit part. The internal current source is used to determine whether the actual state of the battery is open circuit. For example, when C3 is disconnected, the readings of the two batteries B3 and B4 connected to C3 are close to 0. At this time, the host can turn on the current source set by LTC6802 between AD and V- through a command. If C3 is actually in a disconnected state, when the data is read again, B3 is 0, and B4 is close to B3 + B4 + 0.5V.

Figure 3 Open circuit detection circuit

In order to improve the accuracy of AD, a filtering link is usually added to the external circuit. As shown in Figure 4, when an RC filtering link is added externally, the open circuit part will not produce a 0 voltage value, because the AD input impedance is too large to discharge the capacitor connected to the input pin. When C3 is disconnected, after several measurement cycles, the AD input resistor charges CF3 and CF4. The potential of C3 is close to the midpoint between C2 and C4. At this time, the measured values ​​of B3 and B4 are not the actual values. If the internal 100uA current source is enabled at this time, the potential of C3 will be pulled down, the value of B3 is close to 0, and the value of B4 is close to the full scale. The best way to detect whether the CN point is open is to compare the voltage of BN + 1 battery before and after enabling the internal 100uA current source. If the voltage values ​​measured twice differ by more than 0.2 volts, it can be determined that the CN point is open.

Figure 4 Open circuit detection circuit with external filtering 2.4.4 Overtemperature protection

The electrical characteristics of the chip ensure that the chip can work normally below 85 degrees. When the core temperature exceeds 105 degrees, the measurement accuracy gradually decreases. When it approaches 150 degrees, the chip is damaged and cannot work normally.

Therefore, the recommended maximum core temperature of the chip is 125 degrees.

To protect the chip from damage due to overheating, an over-temperature protection circuit is included in the chip. The chip may overheat when the discharge switch is turned on to discharge the battery with a large current or when current mode communication is frequently used. The overheating phenomenon is more serious when a large voltage is applied to the positive and negative terminals of the chip power supply or the overall thermal conductivity of the system is poor.

The over-temperature protection circuit works in non-standby mode. When the chip detects that its temperature is greater than 145 degrees, the value in the command register will be reset to the default value, the discharge switch will be turned off, the A/D conversion will be stopped, and the current communication mode will be interrupted. The THSD bit in the temperature register is set high, and the value of the THSD bit is automatically cleared after being read.

Since overtemperature will interrupt normal operation of the chip, the chip temperature should be monitored in real time using an internal temperature monitor.

2.5 Registers and Control Commands

LTC6802 has four register groups: command register, voltage register, temperature register, and flag register. The corresponding registers are accessed through corresponding read and write commands. By configuring the command register, parameters such as the number of battery cells to be measured, battery voltage measurement time, overvoltage and undervoltage thresholds, and discharge switch status can be set. The specific contents of the command register are shown in Table 1.

Table 1 LTC6802 command register

2.6 Interface Timing

The chip is accessed through the SPI serial interface. The access timing is shown in Figure 5. CSB I is the serial port enable terminal, which is controlled by the host; it is pulled low at the beginning of a data transmission and pulled high again at the end of the transmission. SCLK is the serial port clock signal, which is controlled by the host. When writing commands, the SDI input must remain stable at the rising edge of SCLK. When reading data, SDO is valid at the rising edge of SCLK.

If there is no clock signal input for two seconds, the watchdog timer output will be set low, the command register will be reset, and the chip will enter a low-power standby mode. At this time, all chip functions except the serial interface and voltage reference source are disabled.

Figure 5 SPI access timing diagram

3 Voltage Detection Application Examples

3.1 Hardware Design

Since the chip can work in a cascade mode, it is necessary to set the corresponding pin high or low according to the position of the chip in the series group during hardware design. If the chip is in the lowest position and is directly connected to the CPU, the chip uses voltage mode communication, Vmode is connected to V reg, and other chips in the series group use current mode communication, and the corresponding Vmode is connected to V -. When the chip is in the highest position of the cascade, TOS is connected to V reg, and TOS of other chips in the series group is connected to V -, allowing data to be transmitted through the SDO I pin. The connection method between the chip and the CPU is shown in Figure 6.

Figure 6 LTC6802 application example

3.2 Software Design

The chips can work in cascade. When the chips are used in series, commands are written to the chips in descending order according to the order of the chips in the series group. When reading data, the data is read out in descending order according to the order of the chips in the series group.

The microcontroller can access the timing by simulating the SPI interface through the IO port, which makes the application more flexible. The following is the voltage measurement achieved by operating the LTC6802. In the battery management system application, the Freescale S12 series microcontroller is used to access the sensor through the IO port simulating SPI. To illustrate the problem, two main operation program lists are given:

ccs68002( );

w rcmd_ltc( 0x01) ; Configure command register

w rcmd_ltc( 0x00) ;

w rcmd_ltc( 0x00) ;

w rcmd_ltc( 0x00) ;

w rcmd_ltc( 0x00) ;

w rcmd_ltc( 0x00) ;

w rcmd_ltc( 0x00) ;

scs68002( ) ;

de lay( 1) ;

ccs68002( ) ;

w rcmd_ltc( 0x10) ; Start voltage conversion

scs68002( ) ;

de lay( 1) ;

ccs68002( ) ;

w rcmd_ltc( 0x04) ; Read voltage data

for ( i = 0; i < 19; i + + )

{

temp = rddata_ltc();

}

scs68002( ) ;

void w rcmd_ltc(uchar cmd) write command

{

Byte ;i

csclk68002( ) ;

for( i = 0; i < 8; i + + )

{

if( ( cmd&0x80) == 0x80)

{

sdo68002( );

}

else

{

cdo68002( ) ;

}

ssclk68002( ) ;

cmd= cmd<<1;

csclk68002( );

}

}

Byte rddata_ ltc( void) Read command

{

Byte ,i res= 0;

csclk68002( );

for ( i = 0; i < 8; i + + )

{

res = res < < 1;

ssc lk68002( );

if( d i68002 == 1)

res= res|1;

csclk68002( );

}

return res;

}

4 Conclusion

In actual applications, the time to measure all batteries is 13ms, and the voltage measurement error is within 10mV, which fully meets the accuracy requirements of the battery management system. The high integration, high measurement accuracy, fast measurement time, and low power consumption of LTC6802 have made it well used in battery management systems for electric vehicles.