LMD18245_06 Datasheet, PDF (10/21 Pages) National Semiconductor (TI) – 3A, 55V DMOS Full-Bridge Motor Driver

Functional Descriptions (Continued)
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FIGURE 3. Decay Paths for Initial Winding Current During a Change in the Direction of the Winding Current
THE CURRENT SENSE AMPLIFIER
Many transistor cells in parallel make up the DMOS power
switches. The current sense amplifier (Figure 5 ) uses a
small fraction of the cells of both upper switches to provide a
unique, low-loss means for sensing the load current. In
practice, each upper switch functions as a 1x sense device
in parallel with a 4000x power device. The current sense
amplifier forces the voltage at the source of the sense device
to equal that at the source of the power device; thus, the
devices share the total drain current in proportion to the
1:4000 cell ratio. Only the current flowing from drain to
source, the forward current, registers at the output of the
current sense amplifier. The current sense amplifier, there-
fore, sources 250 µA per ampere of total forward current
conducted by the upper two switches of the power bridge.
The sense current develops a potential across R_Sthat is
proportional to the load current; for example, per ampere of
load current, the sense current develops one volt across a
4 kΩ resistor (the product of 250 µA per ampere and 4 kΩ).
Since chopping of the load current occurs as the voltage at
CS OUT surpasses the threshold (the DAC output voltage),
R_S sets the gain of the chopper amplifier; for example, a
2 kΩ resistor sets the gain at two amperes of load current
per volt of the threshold (the reciprocal of the product of
250 µA per ampere and 2 kΩ). A quarter watt resistor suf-
fices. A low value capacitor connected in parallel with R_S
filters the effects of switching noise from the current sense
signal.
While the specified maximum DC voltage compliance at CS
OUT is 12V, the specified operating voltage range at CS
OUT is 0V to 5V.
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