effectively extend battery life.Charging and protection IC is the separation of the occupied area and the peripheralcircuit complexity. Currently, the market has not yet real, cha rged with the protectionfunction is integrated on a single chip.For th is problem,design a set of batterycharging and protection functions in one IC is very necessary.2.System design and considerationsThe system mainly includes two parts:the b attery charger module and theprotection module. Of great significance for the battery as st andby power use of theoccasion, It can ensure that the external power supply to the battery-powered, but alsoin the battery overcharge, over-current and an external power supply is disconnectedthe battery is to put the state to provide protection ,the charge and protection rolledf the battery is not thesame,so the battery over discharge current detection should be treated separately.When the battery is charging the state a long time, would sev erely reduce the capacityof the battery and shorten battery life. When the battery is the time of discharge statusexceeds the allotted time, the battery, the battery voltage is too low may not be able torecharge, making the battery life is lower.Based on the above, the charge on the life of maintenance-free lead-acid batterieshave a significant impact,while the battery is a lways in good working condition,battery protection circuit must be able to detect the normal working condition of thebattery and make the action the battery can ne ver normal working state back tonormal operation, in order to achieve the protection of the ba ttery.3.Units modular design3.1The charging modulethe constant charging current of the charger output in this state.And the chargercontinuously monitors the voltage across the battery pack, the battery power has beenrestored to 70% to 90% of the released capacity when the bat tery voltage reaches theswitching voltage to charge conversion voltage V sam cha rger moves to the state ofcharge. In this state, the charger output voltage is increased to overcharge pressure V ocis due to the charger output voltage remains constan t,so the charging current is acontinuous decline. Current down to charge and suspend the cur rent Ioct, the batterycapacity has reached100%of rated capacity,the charg er output voltage drops to alower float voltage V F.3.2Protection ModuleChip block diagram of the internal protection cir cuit shown in Figure 3.Thecircuit includes control logic circuit,sampling circuit, overcharge detection circuit,over-discharge detection comparator,overcurrent detection comparator,load short-is high,and charging controlterminal CO is low, the chip is in charging mode. When the s upply voltage of the chipunder the overdischarge detection voltage (V dd<V dl,), then th e battery is dischargedstate,this state remains the overdischarge delay time tdl chip will be prohibited todischarge the battery at this time The charge c ontrol side CO is high,while thedischarge control terminal DO is low, the chip is in discharge mode.4.Circuit DesignTwo charge protection module structure diagram, the circuit can be divided intofour parts:the power detection circuit(under-voltage detection circuit),part of thebias circuit(sampling circuit,the reference circuit an d bias circuit),the comparator(including the overcharge detection /overdischarge detection compa rator, over-currentdetection and load short-circuit detection circuit) and the logi c control part.This paper describes the under-voltage detection circuit (Fig ure 5), and gives thethe comparator, when the normal working of the power supply v oltage, the invertingterminal of the voltage detection is lost tothe inverting terminal voltage of thecomparator is greater than V+. Comparator output is low, M14cu toff, feedback circuitdoes not work; undervoltage occurs, the voltage divider of R1, R2, R3, reaction is moresensitive, lost to the inverting input voltage is less than V when the resistor divider,the comparator the output voltage is high,this signal will be M14open,the voltageacross R into M at both ends of the saturation vo ltage close to0V,thereby furtherdriving down the R1> R2, the partial pressure of the output v oltage, the formation ofthe undervoltage positive feedback.Output,underv oltage lockout,and plays aprotective role.5.Simulation results and analysisThe design of the circuit in CSMC 0.6μm in digital CMO S process simulation。