This video is the 7th of a series of tutorials discussing
the LX7720 (L-X-seventy-seven-twenty) radiation-hardened motor driver for space.
First, here's a reminder of what the LX7720 actually is.
The LX7720 is a mixed-signal peripheral which works with an FPGA (F-P-G-A) or a microcontroller
to build a complete closed-loop brushless DC motor controller or stepper motor system.
The first two videos outlined the LX7720 at a system level
and then dived into a detailed discussion of the half-bridge FET (phet) drivers.
Videos 3 and 4 introduced the LX7720's four current sense amplifiers and showed how to
use a current sense amplifier in each the 3 common current measurement topologies used with motors.
Video 5 discussed separating a half-bridge into independent low-side and high-side drivers,
and how to use current sensing in these cases.
Video 6 discussed connecting an LX7720 to
brushless DC and permanent magnet synchronous motors.
This video will discuss connecting an LX7720 to a bipolar stepper motor.
The LX7720 contains four half-bridge driver stages
and four current sense amplifiers, which is enough to drive a two-phase bipolar stepper motor.
Here's a generic two-phase bipolar stepper motor being driven by
four half-bridge drivers. The half-bridges operate in pairs to make two H-bridges.
The LX7720 circuitry is shown simplified, with supplies and decoupling to the current sense and
gate drive sections omitted. The key components are the four half-bridges and the current sensing.
The architecture shown here uses a single current sense per winding,
in series with the common ground path for each H-bridge.
Each H-bridge has bi-directional current sensing in one of its half-bridge outputs.
Current entering one half-bridge from the V-M-P-S motor supply
will exit via the other half-bridge, and a positive voltage will develop
across the current sense resistor proportional to this current.
Since current entering one half-bridge must exit via another half-bridge, only
two current senses are needed with this current sensing architecture.
The half-bridge FETs are shown with their integral body diodes.
Diodes in parallel with the FETs are necessary components to carry commutation currents
when a half-bridge FET switches off. Commutation current flow
after FET switch-off is discussed in the 4th video.
The FET body diodes can be paralleled by additional discrete diodes for
higher switching speed and lower voltage drop.
Motor winding currents normally flow in through a high-side FET from the motor supply V-M-P-S,
and out to the motor ground M-Ground through a low-side FET.
Here's current flow one way through H-bridge.
And here's the alternative current flow.
An alternative current sensing architecture relocates the
current sensing to be at one side of each H-bridge's drive output.
Again, since current entering one half-bridge must exit
via another half-bridge, only two current senses are needed.
However, including the additional current sense circuits provides redundancy, if required.
The two unused current sense stages are shown here, disabled, with their pins tied to ground.
The current sensing is now bidirectional, depending on whether the half-bridge with
the current sense is sourcing current or sinking current.
In this example, the top left half bridge is sinking current,
so the current sense amplifier sees a positive voltage proportional to winding current.
The bottom left half bridge is sourcing current,
so its current sense amplifier sees a negative voltage proportional to winding current.
The LX7720 uses two internal ground domains, shown as the signal ground S-Ground,
and the motor ground M-Ground.
S-Ground is used for the digital and analog signals (the control circuitry).
M-Ground is used for circuitry associated with the external half-bridges - the FET drives,
current sensing, and the motor load itself.
Level shifters manage the crossing of the ground domains. In most applications,
the two grounds S-Ground and M-Ground are joined together directly on the PCB (P-C-B).
The 6th video discusses the ground domains in detail, particularly for high-power
motor designs where the power switching components are on a separate module or PCB.
As a final note, synchronous motors are also
available with more than the two phase-windings demonstrated here.
Since the half-bridge and current sense stages are independent within an LX7720,
multiple LX7720s can be used together to control motors with more than two phases.