Datasheet

LMD18245

SNVS110E –APRIL 1998–REVISED APRIL 2013

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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 9). The bridge consists of four solid state power switches and four diodes connected in an H

configuration. 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

(Figure 9a and Figure 9d ). 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 opposite upper diode

short the winding, and the winding current recirculates and decays slowly towards zero (Figure 9b and

Figure 9e ). At the end of the off-time, the control circuitry turns back ON the sink switch, and the winding current

again increases rapidly towards V

/R (Figure 9a and Figure 9d again). The above sequence repeats to provide

a current chopping action that limits the winding current to the threshold (Figure 9g ). Chopping only occurs if the

winding current reaches the threshold. During a change in the direction of the winding current, the diodes provide

a decay path for the initial winding current (Figure 9c and Figure 9f ). 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.

(a) (b)

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