【Abstract】An expensive digital IC is used in old battery only charger （BOC） and this IC needs separate software. For simplify the design and reduce the cost， we find the existing analogy charging ICs and select the most popular four from them， including Mitsumi， Freescale， TI and Analogic Tech， we make a detailed comparison and select the best one， based on this IC’s feature and our requirements， A detailed design evaluation is presented， including the design of the input over voltage protection （OVP）， output OVP and over temperature protection （OTP）， the WCA （Worse Case Analysis） of this OTP. After the improvement， this new BOC has lower cost， simpler topology， more efficient power consumption.

1.Introduction

In the old BOC， an expensive digital IC is used， it needs separate calibration and software reset circuit， it also needs separate software. All of them make the controller complex and expensive. Furthermore， it uses two three terminal voltage regulator to provide the +3V power， this will increases the quiescent and standby current. It is a key concern for our green power requirement.

In order to make the controller higher integrate， simpler， cheaper and more efficient， we try to use analogy charging IC instead of the digital IC， we find the existing charging ICs and select the top four， including Mitsumi， Freescale， TI， and Analogic Tech. After a detailed comparison， we eventually choose the TI charging IC BQ24086 which is the 1A single chip Li-Ion/Li-Pol charge management IC with thermal regulation.

2.Comparison of the Charging ICs

We compared the four kinds of charging ICs： MM1663 （Mitsumi）， MC34674 （Freescale）， BQ24086 （TI）， AAT3685 （Analogic Tech） in the following ten items： Thermal risk level， PCB Layout size， USB compatible， LED scheme， Operation current， OTP， short protection， output OVP， charging timer and IC price.

For the thermal risk， because MM1663 has a separate MOSFET for its control， it has the best thermal performance， the other three ICs are the same， they all combine the MOSFET in the IC； For the PCB layout size， MM1663 need a separate MOSFET， so its PCB layout size is the largest， the others are the same；

For the USB compatible， AAT3685 has input UVP （Under voltage protection）， it can protect the USB equipment while the input charging current bigger than 500mA， the others all have no this function；

For the LED scheme， MM1663 and BQ24086 have the simple but completed LED indication， The indication of MC34674 is too simple， it has no charging completed indication， AAT3685 has a flashing indication when no battery， but it is conflict with the green requirement， it can increase the standby current；

For the operation current， MM1663 and BQ24086 have large IC current consumption， the LED of AAT3685 shall be flashing when no battery， it may increase the operation current， MC34674 has the best performance；

For the OTP， because the thermistor in battery has +/-5% tolerance， all of them cannot meet the OTP（Over temperature protection） requirement although they all have the OTP；

For the short protection， MM1663 and BQ24086 have a short protection， MC34674 and AAT3685 have a Pre-charge phase at this situation；

For the Output OVP （Over voltage protection）， MM1663 has a 4.3～4.4V OVP which can meet our requirement， the other three cannot meet this requirement， they need additional circuit to implement this；

For the charging timer， MC34674 and AAT3685 have no timer， MM1663 and BQ24086 have a modified timer；

For the IC price， BQ24086 have the lowest cost.

Basing on these comparisons， we choose the TI charging IC BQ24086 although it has some defects. First item is the thermal issue， we can have large ground pad under the charging IC and also dig several holes in the PCB； Second is the USB compatible， there are totally two kinds of USB ports： 500mA and 100mA， the maximum input current of the BOC can be controlled within 500mA， so it is safe for the 500 mA USB port， and for the 100 mA port， it is very few and also can’t lead to any safety issue except for deficient charging. Third item is the high operation current， the standby current of this new BOC is less than 8 mA if we use BQ24086， it is much less than 18 mA of our old BOC， it can be accepted by the green requirement. Fourth item is the output OVP. We just need add an amplifier circuit to implement this function. The detailed design will be described below. Fifth item is the temperature protection， because all the four charging ICs can not meet our requirement （Operation range is 0+/-3 to 45+/-3degree C） although they all haveOTP function， we need design the temperature protection circuit for this BOC， the detailed design will be described below.

3.New Design for the BOC

3.1Design of the OVP

Actually， we design input and output OVP for the new BOC， the main circuit of this design is shown in Figure1， we connect a N-channel MOSFET （Q1） in series with the main power loop， we also use two NPN transistors in series with the drive gate of Q1， the Q1 may shut down when either of the two OVP is triggered， because of the high accuracy requirement of the input OVP， we use 1% resistors（R1 and R2）， the accuracy of Vref can be got from the TL431 datasheet， so we can calculate the Worse Case Analysis for the OVP range， it is from 5.43 to 5.60V， for the output OVP， it is required within 4.242 to 4.4V by our safety team， we use 0.5% resistors（R3 and R4）， we also can easily get the tolerance of this OVP， it is from 4.28 to 4.36V[1][2].

Figure 1

3.2Design of the OTP

Figure 2

The design of the OTP is the key part of this BOC， the WCA （Worse Case Analysis） of this is also the most difficult calculation. Figure2 is the main circuit of the OTP， it use two amplifiers to make two hysteresis comparators， in order to meet the operation range （0+/-3 to 45+/-3degrees C）， we use 1% accuracy resistors for the R5， R6， R7， R9 and R10， and also 0.5% accuracy resistor for the R8， We will give a detailed WCA in next section. The same to the section3.1， two NPN transistors are used to control the /CE pin of the charging IC， they are paralleled connected as showed in the Figure2， the output of the two comparators are connected to the base of the Q3A and Q3B， the voltage of R12 is connected to the charging IC.

3.3Design and Setting of Charging IC

Figure3

The setting of charging IC is relatively simple， you can get the detailed setting method from the IC datasheet[3]， but there are some experience to improve the performance of this charging IC， firstly， you can add 1uF capacitor with the /CE pin， this is very helpful for improving the ESD capacity of this IC. Second， the C2， which is between output and ISET， shall be evaluated by your actual project， it is helpful for current accuracy in the constant current mode， but it may impact the IC’s dynamic performance. In this BOC， we do not connect this capacitor. Third， the resistance of charging timer R14 shall not be too large， the output filter capacitor C3 cannot be too small， otherwise the IC may work abnormally when the impedance of the output trace is big. In our BOC， if we insert battery into the BOC very slow， the LED may be flashed red or solid green， especially for the nearly full battery.

4.WCA of OTP

4.1OTP analysis method

Because the calculation process for the high and low temperature protection is almost the same， We just give a detailed analysis for the low temperature in this paper. Figure4 below is the simplified topology of the low temperature protection， Rx is the sum of the R6 and R7. We will assume that we know the range of R8， R15， R9， R （Thermistor resistance within -3 to 3 deg. C）， then we can calculation the range of R and Rx， if their resistance are within +/-1% accuracy， we can say the low temperature protection can meet -3 to 3 deg. C requirement. After we fix the R5 and Rx， we can also calculate the range of the thermistor resistance when our BOC recovery from the low temperature protection.

4.2Calculation for Low OTP

When the temperature becomes cold and the OTP is triggered， the equation below can be achieved[4]：

After a simple modification， we can get another equation below[5]：

Figure4

In the right side of the equation， if it is the maximum value， the R5 shall be the minimum value and Rx shall be the maximum one， in the left side of the equation， if it is maximum value， the R8 and R15 shall be minimum value， the R and R9 shall be maximum value. If we assume the sum of R5 and Rx are 102.5， below equation can be achieved[6]：

Because R8， R9， R15 and R_cold under -3～3degree C are all available， we can get the result of the and Rx_max easily with the MathCAD software： Rx_max=77.196Kohm

In the same theory of the calculation， we can get another couple of R5 and Rx：

Rx_max=7.3856Kohm

In our actual circuit， we choose R5 as 26.1Kohm+/-1% and Rx as 76.4ohm+/-1%， they are within the range above， so our design can meet the -3～3degree C protection requirement.

4.3Calculation for Low OTP Recovery

When the temperature rise， the OTP will be released， below equation also be achieved：

From these equations， we can get the maximum recovery resistance of the thermistor in the battery：

R_re_cold_min=27.463kohm

In the same theory of calculation， we also can get the minimum recovery resistance of the thermistor：

R_re_cold_max=28.297kohm

So the range of the thermistor for the recovery from the cold temperature protection is 27.463 to 28.297Kohm， check with the thermistor resistance VS. temperature table， the recovery temperature is 0 to 5 degrees C， it can meet the requirement.

With the same method， we can get the high temperature protection result： high protection temperature is 41 to 48 deg. C， the recovery temperature is 39 to 43 deg. C.

In Table3， low protection temperature， low recovery temperature， high protection temperature and high recovery temperature are showed. All the three samples can meet our calculation results.

5.Conclusion

This BOC has a good performance and can meet requirement. Compared to the previous BOC， we will easily find that it not only has lower cost and simpler electrical， but also better power consumption.

References：

[1]Theodore F. Bogart Jr， Jeffrey S . Bersley ， Guillermo Rico. Electronic Devices and circuits. American：2006.

[2]Adel S. Sedra and Keneth C.Smith. Microelectronic Circuits. Oxford University Press.Inc. 1998.

[3]BQ24085 data sheet.

[4]Donald A. Neamen. Electronic Circuits Analysis and Design. McGraw-Hill Companies，Inc.2001.

[5]Sergio Franco. Desgign with Operational Amplifiers and Analog Integrated Circuits. American：2004.

[6] D.H.Sheingold，ed. Nonlinear Circuits Handbook. Analog Devices. Norwood. MA. 1974