THE CURRENT SENSE AMPLIFIER

Many transistor cells in parallel make up the DMOS power

switches. The current sense amplifier (

Figure 4 ) uses a

small fraction of the cells of both upper switches to provide a

unique, low-loss means for sensing the load current. In prac-

tice, each upper switch functions as a 1x sense device in

parallel with a 4000x power device. The current sense ampli-

fier forces the voltage at the source of the sense device to

equal that at the source of the power device; thus, the de-

vices 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

portional to the load current; for example, per ampere of load

current, the sense current develops one volt across a 4 k

Ω

resistor (the product of 250 µA per ampere and 4 k

Ω). 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 k

Ω

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 k

Ω). A quarter watt resistor suffices. A low

value capacitor connected in parallel with R

fects 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.

THE DIGITAL-TO-ANALOG CONVERTER (DAC)

The DAC sets the threshold voltage for chopping at

V

of the binary number applied at M4 through M1, the digital in-

puts of the DAC. M4 is the MSB or most significant bit. For

applications that require higher resolution, an external DAC

can drive the DAC REF input. While the specified maximum

DC voltage compliance at DAC REF is 12V, the specified op-

erating voltage range at DAC REF is 0V to 5V.

THE COMPARATOR, MONOSTABLE AND WINDING

CURRENT THRESHOLD FOR CHOPPING

As the voltage at CS OUT surpasses that at the output of the

DAC, the comparator triggers the monostable, and the

monostable, once triggered, provides a timing pulse to the

control logic. During the timing pulse, the power bridge

shorts the motor winding, causing current in the winding to

recirculate and decay slowly towards zero (

Figure 1b and

Figure 1e again). A parallel resistor-capacitor network con-

nected between RC (pin #3) and ground sets the timing

pulse or off-time at about 1.1 RC seconds.

Chopping of the winding current occurs as the voltage at CS

OUT exceeds that at the output of the DAC; so chopping oc-

curs at a winding current threshold of about

(V

DS011878-14

FIGURE 4. The Source Switches of the Power Bridge and the Current Sense Amplifier

www.national.com

8