LMD18245_06 Datasheet, PDF (6/21 Pages) National Semiconductor (TI) – 3A, 55V DMOS Full-Bridge Motor Driver

Functional Descriptions
TYPICAL OPERATION OF A CHOPPER AMPLIFIER
Chopper amplifiers employ feedback driven switching of a
power bridge to control and limit current in the winding of a
motor (Figure 1). The bridge consists of four solid state
power switches and four diodes connected in an H configu-
ration. Control circuitry (not shown) monitors the winding
current and compares it to a threshold. While the winding
current remains less than the threshold, a source switch and
a sink switch in opposite halves of the bridge force the
supply voltage across the winding, and the winding current
increases rapidly towards V_CC/R (Figure 1a and Figure 1d ).
As the winding current surpasses the threshold, the control
circuitry turns OFF the sink switch for a fixed period or
off-time. During the off-time, the source switch and the op-
posite upper diode short the winding, and the winding cur-
rent recirculates and decays slowly towards zero (Figure 1b
and Figure 1e ). At the end of the off-time, the control cir-
cuitry turns back ON the sink switch, and the winding current
again increases rapidly towards V_CC/R (Figure 1a and Fig-
ure 1d again). The above sequence repeats to provide a
current chopping action that limits the winding current to the
threshold (Figure 1g ). Chopping only occurs if the winding
current reaches the threshold. During a change in the direc-
tion of the winding current, the diodes provide a decay path
for the initial winding current (Figure 1c and Figure 1f ).
Since the bridge shorts the winding for a fixed period, this
type of chopper amplifier is commonly referred to as a fixed
off-time chopper.
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