lmd18245-mdc National Semiconductor Corporation, lmd18245-mdc Datasheet - Page 11

• Manufacturer Part Number: lmd18245-mdc
• Description: 3a, 55v Dmos Full-bridge Motor Driver
• Manufacturer: National Semiconductor Corporation

Functional Descriptions
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
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
The R
the maximum rated current of the LMD18245, with D = 15
and V
DAC REF
(5.00V x 15/16) ÷ ((250 x 10
(V
S
DAC REF
DAC REF
value required to set the winding current threshold at
x D/16, where D is the decimal equivalent (0–15)
of 5.00V would be:
x D/16) ÷ ((250 x 10
FIGURE 5. The Source Switches of the Power Bridge and the Current Sense Amplifier
−6
) x 6.25 k )) = 3.00A
−6
) x R
S
(Continued)
)) amperes.
11
The resulting typical DAC programmable current limit values,
for different values of R
FIGURE 4. D to A winding current thresholds for
10
11
12
13
14
15
D
0
1
2
3
4
5
6
7
8
9
18.75 k
0.00A
0.07A
0.13A
0.20A
0.27A
0.33A
0.40A
0.47A
0.53A
0.60A
0.67A
0.73A
0.80A
0.87A
0.93A
1.00A
R
S
=
V
REF DAC
S
, would be:
9.375k
= 5.00V
0.00A
0.13A
0.27A
0.40A
0.53A
0.67A
0.80A
0.93A
1.07A
1.20A
1.33A
1.47A
1.60A
1.73A
1.87A
2.00A
R
S
=
01187814
6.250 k
0.00A
0.20A
0.40A
0.60A
0.80A
1.00A
1.20A
1.40A
1.60A
1.80A
2.00A
2.20A
2.40A
2.60A
2.80A
3.00A
R
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S
=