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UC2637 Datasheet(PDF) 4 Page - Texas Instruments |
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UC2637 Datasheet(HTML) 4 Page - Texas Instruments |
4 / 7 page UC1637 UC2637 UC3637 MODULATION SCHEMES Case A Zero Deadtime (Equal voltage on Pin 9 and Pin 11) In this configuration, maximum holding torque or stiffness and position accuracy is achieved. However, the power in- put into the motor is increased. Figure 3A shows this con- figuration. Case B Small Deadtime (Voltage on Pin 9 > Pin 11) A small differential voltage between Pin 9 and 11 provides the necessary time delay to reduce the chances of mo- mentary short circuit in the output stage during transi- tions, especially where power-amplifiers are used. Refer to Figure 3B. Case C Increased Deadtime and Deadband Mode (Voltage on Pin 9 > Pin 11) With the reduction of stiffness and position accuracy, the power input into the motor around the null point of the servo loop can be reduced or eliminated by widening the window of the comparator circuit to a degree of accep- tance. Where position accuracy and mechanical stiffness is unimportant, deadband operation can be used. This is shown in Figure 3C. PWM Comparators Two comparators are provided to perform pulse width modulation for each of the output drivers. Inputs are un- committed to allow maximum flexibility. The pulse width of the outputs A and B is a function of the sign and ampli- tude of the error signal. A negative signal at Pin 10 and 8 will lengthen the high state of output A and shorten the high state of output B. Likewise, a positive error signal re- verses the procedure. Typically, the oscillator waveform is compared against the summation of the error signal and the level set on Pin 9 and 11. Figure 1. Oscillator Setup Figure 2. Comparator Biasing Output Drivers Each output driver is capable of both sourcing and sinking 100mA steady state and up to 500mA on a pulsed basis for rapid switching of either POWERFET or bipolar tran- sistors. Output levels are typically -VS + 0.2V @50mA low level and +VS - 2.0V @50mA high level. Error Amplifier The error amplifier consists of a high slew rate (15V/ µs) op-amp with a typical 1MHz bandwidth and low output im- pedance. Depending on the ±VS supply voltage, the com- mon mode input range and the voltage output swing is within 2V of the VS supply. Under-Voltage Lockout An under-voltage lockout circuit holds the outputs in the low state until a minimum of 4V is reached. At this point, all internal circuitry is functional and the output drivers are enabled. If external circuitry requires a higher starting volt- age, an over-riding voltage can be programmed through the shutdown terminal as shown in Figure 4. 4 |
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