LMD18245_06 Datasheet, PDF (11/21 Pages) National Semiconductor (TI) – 3A, 55V DMOS Full-Bridge Motor Driver

Functional Descriptions (Continued)
THE DIGITAL-TO-ANALOG CONVERTER (DAC)
The DAC sets the threshold voltage for chopping at
V_{DAC REF} x D/16, where D is the decimal equivalent (0–15)
of the binary number applied at M4 through M1, the digital
inputs 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
operating 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
occurs at a winding current threshold of about
(V_{DAC REF} x D/16) ÷ ((250 x 10^−6) x R_S)) amperes.
The R_S value required to set the winding current threshold at
the maximum rated current of the LMD18245, with D = 15
and V_{DAC REF} of 5.00V would be:
(5.00V x 15/16) ÷ ((250 x 10^−6) x 6.25 kΩ)) = 3.00A
The resulting typical DAC programmable current limit values,
for different values of R_S, would be:
R_S =
R_S =
R_S =
D
18.75 kΩ
9.375kΩ
6.250 kΩ
0
0.00A
0.00A
0.00A
1
0.07A
0.13A
0.20A
2
0.13A
0.27A
0.40A
3
0.20A
0.40A
0.60A
4
0.27A
0.53A
0.80A
5
0.33A
0.67A
1.00A
6
0.40A
0.80A
1.20A
7
0.47A
0.93A
1.40A
8
0.53A
1.07A
1.60A
9
0.60A
1.20A
1.80A
10
0.67A
1.33A
2.00A
11
0.73A
1.47A
2.20A
12
0.80A
1.60A
2.40A
13
0.87A
1.73A
2.60A
14
0.93A
1.87A
2.80A
15
1.00A
2.00A
3.00A
FIGURE 4. D to A winding current thresholds for
V_{REF DAC} = 5.00V
01187814
FIGURE 5. The Source Switches of the Power Bridge and the Current Sense Amplifier
11
www.national.com