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AN603 vishay siliconix document number: 70574 08-jun-00 www.vishay.com faxback 408-970-5600 1 designing with the si9978dw configurable h-bridge controller wharton mcdaniel currently, there are a number of fully integrated h-bridges on the market. both bipolar and mos technologies have been used to create these parts. however, all of them suffer from problems such as limited current handling capability, large package size, and difficulty of assembly. the si9978dw addresses these problems with an architecture that allows flexible current handling capability, small size, and ease of assembly. instead of trying to integrate the power devices with the controller on a single piece of silicon, the si9978dw approach is to separate the controller from the mosfets. the benefit is efficient manufacture of both the controller and the mosfets, the flexibility to select the optimum mosfet for the application, and the ability to use surface-mount parts for both the controller and mosfets which makes assembly easier. altogether this means a low cost h-bridge solution. the si9978dw is a monolithic controller designed to be used with little foot power mosfets to create an all-n-channel h-bridge or two separate half bridges. in addition to this functional flexibility, the dual function allows standardization of components and reduces inventory costs. the si9978dw features integral high-side drive circuitry and an internal voltage regulator, which a llows operation over a 20- to 40-v dc input voltage range. protection features include cross-conduction protection, current limiting, and undervoltage lockout. the fault outputs indicate when undervoltage or overcurrent shutdown has occurred. the si9978dw is packaged in a 24-pin wide body soic. the si9978 has two modes of operation, full h-bridge, and independent half-bridge. the mode of operation determines the function of the input pins and the fault outputs. with the mode pin at logic 1, which is the default condition, the si9978 functions as a full h-bridge. with the mode pin at logic 0, the si9978dw functions as two independent half-bridges. the definitions of the control inputs depend on the mode of operation. all control inputs are pulled up to v dd . the fault outputs are open drain. truth tables have been provided for each mode of operation. figure 1. the si9978 is made up of the control logic, the gate drive outputs, v dd regulator, and protection circuitry. dir/in a qs/in b mode brk fault /fault a cl /fault b v+ gt a + il a + pwm/en b en/en a v dd r a /c a v dd input logic v dd v dd v dd v dd v dd v dd cap a cap b + il b + r b /c b 24 1 3 5 4 6 7 2 8 9 11 12 21 13 14 high-side u.v. lockout bootstrap reg. charge pump s a 22 gt b 17 bootstrap reg. charge pump s b 18 cap a 23 cap b 19 gb a 20 gb b 16 v dd v dd 15 gnd one shot one shot low-side u.v. lockout low-voltage regulator
AN603 vishay siliconix www.vishay.com faxback 408-970-5600 2 document number: 70574 08-jun-00 full h-bridge operation when operating as a full h-bridge, the control inputs become dir, qs, pwm, brk, and en. when connecting the bridge for anti-phase operation, the dir input is driven by the control system pwm signal. when connected for sign-magnitude operation, the dir input controls direction, and the system pwm signal drives the pwm input. independent half-bridge operation in the independent half-bridge mode, the control inputs become the independent controls for each half-bridge, ina, ena, inb, enb. the brk input is not functional in this mode of operation. gate drive outputs each half-bridge output is driven by a pair of n-channel mosfets. these are controlled by a low- and a high-side gate driver. they have been designed to drive a 600-pf load with a 110-ns rise time and a 50-ns fall time. the low-side gate is driven directly from the commutation logic and is powered by v dd . this means that v dd must be decoupled with a 1- f capacitor; otherwise the turn-on surge current can cause v dd to drop to the level of an undervoltage condition. the high side is a floating circuit powered from a combination bootstrap/charge pump supply. the bootstrap capacitor is charged to v dd when ever the low-side mosfet is turned on. at all other times, the charge pump keeps the bootstrap capacitor charged, replacing the charge used in powering the high-side circuitry and in turning on the mosfet. the value of the bootstrap capacitor is a function of the mosfet being driven. the bootstrap voltage should not drop more than 1 v as the result of a mosfet turn-on. for a 60-v dual n-channel mosfet like the si4946ey, 30 nc (qg) is required for turn-on at a vgs of 10 v. using the equation c = qg/v gs , 30 nf is required to provide sufficient charge for turn-on. to meet the criterion of dropping only 1 v at turn-on, the capacitor needs to be 10 times as large, making the equation c boot = 10(qg/v gs ). this makes the minimum value of c boot equal to 0.039 f. table 3 gives minimum recommended values for several mosfets that might be used with the si9979. this minimum recommended value is one standard value above the minimum calculated value. mode dir/ in a en/ en a qs/ in b pwm/ en b brk il a + il b + gt a gb a gt b gb b cl / fault b fault/ fault a condition 1 1 1 1 0 l x h l l 1 1 nl 1 1 1 0 0 l x l l 1 1 normal oti 1 0 1 1 0 l x l h l 1 1 operation 1 0 1 0 0 l x l l 1 1 1 x 1 x x 1 l x l h l h 1 1 brake 1 x 0 x x x l x l l l l 1 1 disable 1 x 1 x x 0 x l l l l over-current 1 x x x x x x x l l l l 1 0 undervoltage on v dd mode dir/ in a en/ en a qs/ in b pwm/ en b brk il a + il b + gt a gb a gt b gb b cl / fault b fault/ fault a condition 0 1 1 x 0 x l l h l l l 1 1 nl 0 0 1 x 0 x l l l h l l 1 1 normal oti 0 x 0 1 1 x l l l l h l 1 1 operation 0 x 0 0 1 x l l l l l h 1 1 0 x 1 x x x x l l x x 1 over-current on a 0 x x x 1 x x x x l l 1 over-current on b 0 x x x x x x x l l l l 0 0 undervoltage on v dd AN603 vishay siliconix document number: 70574 08-jun-00 www.vishay.com faxback 408-970-5600 3 part number r ds(on) qg @ v gs = 10 v (nc) minimum recommended c boot ( f) si4946 0.055 30 0.039 si9956 0.10 7 0.01 |