The Design Of An Analog Switch With Low On-state Resistance

Along with the development of the integrated circuit techniquefrom large scale integration (LSI), super large scale integration (VLSI)to the extremely high speed integrated circuit (VHSIC), the physicalvolume of various electronics equipments decrease .The biggest effectthat the decrease of the size affects imitating the electric circuit to isthe following decrease of the power supply electric voltage. Theimitating switch, as the necessary part of the imitating electric circuit,requests the more accuracy.At first, the analog switch is mechanical contact point switch, andthen it develops to the semi-conductor switch. As MOSFET (metaloxide semiconductor field-effect-transistor) creates the condition forthe integration of the imitating switch, it is made to become the bestchoose of the portable electronic equipments switch.Most portable electronic equipments work in standard electricvoltage (3.3 V/5 V), so the dual analog switches in this paper will beanalyzed and designed with these two electric voltages. The dualswitches of this electric circuit can work separately and can also worktogether, and the whole electric circuit can also be a mold piece togather go to electronics machine piece that converts the type of themachine such as the power supply.As NMOS Transistors have following characteristics: a) the speedof the switch is quick;b) no second breakdown and safety work area isbig;c) the driving power is small;d) comparing with a BJT transistor,the variational voltage of grid will not affect the voltage of source anddrain;e) the current drive and transconductance are higher than PMOS,the dual switches that we design all adopt the NMOS as the switch.When switches work normally, they must operate in the saturationthat is assuring VG S -VT H >VD S. However, when the NMOS is used asswitch, the voltage of source and drain is almost equal, alsonamely VG >VI N +VT H, so it needs to provide the big electric voltage forthe grid. The step-up circuit--charge pump must be designed in thehead of the grid. The charge pump of this electric circuit only needs toprovide a big electric voltage for the grid, not to provide the big load,so the size of the mold electric circuit do not need to be very large.And the general charge pump electric can only work in half period ofthe pulse, but the structure using in this paper makes it work in thewhole period. This improves the rising time of the electric chargepump greatly. The charge pump finish its rising power by giving andcutting the capacitance, so a moderate frequency oscillator is theessential condition that the charge pump operates. The oscillatordesigned in this thesis adopts the principle of astable multivibrator andbe biased so as to create the pulse with 50% duty cycle.As the input power of the power supply has a certain scope, thevoltage detector is added at the head of charge pump. By comparingthe power supply voltage, the series of the charge pump are decided,and the static state power consumption is lowed.The ideal on-state resistance of analog switch should be zero, butactually it only can be reduced as far as possible. The bigger the ratioof the switch, the smaller the on-state resistance is. Considering thearea that the actual chip takes up, it is impossible to enlarge the aspectratio of the switch infinitely. Therefore according to the situation thatthe chip takes the circumstance of the load normally, it was carried oncompromised. And the increasing of the size of switch must cause thebig parasitic capacitance. This will effect the speed of MOSFETswitch, so the paper designs the drivers for both switches respectively.They can control the charge and discharge of the switch grid .If thecurrent of charge and discharge is too great, it may cause surge currentand result in the device piece damage easily. Therefore the paper addsrheoscope in the output stage of the switch and adds current limit indriver. Arrived the current of switches feedback to current limit;carryon the comparison with the reference then controls the electric circuitbreak.The paper designs references to provide a reference electricvoltage for each modular circuit, in order to assuring each of them canwork in a suitable situation. The current reference provides twocurrents, a PTAT (proportional to absolute temperature) current and acurrent insensitivity to the temperatures .The voltage references adoptbandgap references, create an accurate voltage of 1.24V.In consideration of accidental situation that the chip may encounter,the paper designs Thermal Shutdown. If the temperature becomes over130 °C, the switch will close and not open until the temperature low to120°C.The whole switch consists of a number of mold pieces as the above.The paper adds EN controlling in each actual modular for reduce thestatic state power consumption.The design of the whole electric circuit is started by the analysissystem structure, making sure the mold piece and related parameterindex signs of demand. Then it crystallizes each mold piece to makesure the electric circuit structure of the each mold piece, the choice ofthe parameter. It adopts the BSIM1 model parameter to carry on thehandicraft calculation, estimating each pipe size, then simulatesthrough Hspice, and decides the actual size of the pipe size.Based on reaching the design index of each model pieces, thepaper combines each statures mold together, carries emulation on thewhole. This electric circuit adopts 0.6um CMOS technology. Theoperating range of power voltage is from 2.9 V to 5.5 V, and eachswitch can supply a current over 150mA and the on-state resistance isbelow 200 mΩ.