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Electronics for CNC apparates

Last update: 31/07/2015

The LS7290 is a production of phases generator IC for stepper motors by LSI-CSI. It can be used to create unipolar or bipolar stepper motor controller, using external mosfet bridges or integrated bridges. It's features are:

 

  • Full/half step mode with pwm chopper current control and 1/4, 1/8, 1/16, 1/32 step mode (microstepping without current control);
  • step/dir mode with enable pin;
  • very low external circuitry.

The use of external bridges made by mosfets allows the construction of controller/drivers even for really big motors. It is quite easy to power the system upto 50-70V with 5-7A/phase.

Too good to be true. In fact, according to the maker and the datasheet, it must be considered an economical replacement of the STM L297 because using the microstepping the chip do not provide current control (and Vref connected to VDD while the Sense pins must be grounded).

 Step Mode M2 M1 M0 Notes 
 FULL  0  0  0  Unipolar and bipolar stepper motor driver using pwm chopping current control. In this case the current is set by the vref voltage
 HALF  0  0  1
 1/4  0  1  0  Microstepping mode without current control. In this case the Vref pin must be connected to VDD while the two sense pins must be connected to the ground
 1/8  0  1  1
 1/16  1  0  0
 1/32  1  0  1
 1/32  1  1  0
 1/32  1  1  1
 

ls7290

All pins are very clearly positioned. The clock can be obtained by an 8 Mhz crystal oscillator or by an r/c. The Phases A/B/C/D and the INH1/ INH2/ must be connected to an AND door (74LS08) and an inverter for the INH1-INH2 pins . If the mosfet bridge is connected directly to the logic door a logic gate mosfet is to be preferred but to get a real power better to use a mosfet driver.

DS0 - DS1 pins will establish the duration of the blanking between the motor steps.

In any case the schematic will clear the connections.

ls7290

This is a very simple and clear schematic to use the LS7290 as microstepping controller. Unfortunately it is not provided a current control. This means that we have to low the power supply or we have to limit the current inserting a power resistor between the connector and the windings of the motor.

In our opinion the microstepping mode of the LS7290 can be really useful if very small stepper motors are used both in unipolar or bipolar configuration.   ("Once the peak motor current causes the voltage across the sense resistors to reach the voltage set by VREF, the outputs are disabled until the next oscillator pulse. The VREF voltage sets the peak current in each motor winding").

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