Datasheet

LMD18245

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SNVS110E –APRIL 1998–REVISED APRIL 2013

Figure 11. 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 12 )

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, therefore, 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

that 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

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 suffices. A low value capacitor connected in parallel with R

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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