Copyright © Parallax Inc. Pololu Dual MC33926 (#28820) v1.0 11/18/2011 Page 3 of 6
For example, if D2 is used for the PWM input (as is typically the case), D1 must be held low to prevent it
from disabling the motor driver. The circuit board provides convenient jumper points for overriding the
motor driver defaults without having to connect extra wires to the module. The current sense and status
flag connections are optional, though monitoring of the status flags can allow detection of latched fault
conditions. The status flags are open-drain output, so the two status flag can be wired together for
applications where I/O pins are scarce and determining which motor driver is experiencing a fault
condition is not necessary. Note that the default state of the enable pin, EN, is LOW, which holds both
motor driver chips in a low-current sleep mode. You will need to hold this pin high (either with an external
connection or via the default-overriding jumper next to the pin) to allow the board to run.
Protection
The MC33926 has under-voltage, over-current, and over-temperature protection. Some protection events
are indicated by the status flag pins (SF).which are active-low pins that can be connected connected to a
single input. If the chip detects an over-current or over-temperature event, the SF is latched LOW and
OUT1 and OUT2 are set to high-impedance. To unlatch the status flag pin toggle the D1, D2 , EN or VIN
lines. The carrier board has a reverse-protection MOSFET for added protection to the motor driver chips.
Real-World Power Dissipation Considerations
The MC33926 motor driver used on this carrier board has a maximum current rating of 5 A continuous.
However, the chip by itself will overheat at lower currents. For example, in our tests at room temperature
with no forced air flow, the chip was able to deliver 5 A for 5 s and 4 A for 18 s before the chip’s thermal
protection started reducing the current. A continuous current of 3 A was right at the over-temperature
threshold; in some tests the thermal protection kicked in after a minute, and in other tests the chip
delivered 3 A for over five minutes without triggering thermal protection. The actual current you can
deliver will depend on how well you can keep the motor driver cool. The carrier’s printed circuit board is
designed to draw heat out of the motor driver chips, but performance can be improved by adding a heat
sink. Our tests were conducted at 100% duty cycle; PWMing the motor will introduce additional heating
proportional to the frequency.
Unlike other H-Bridges, the 33926 has a feature that allows it to gracefully reduce current as the
current exceeds 5 A or as the chip temperature approaches its limit. This means that if you push the chip
close to its limit, you will see less power to the motor, but it might allow you to avoid a complete
shutdown.
