Datasheet
LMD18245
SNVS110E –APRIL 1998–REVISED APRIL 2013
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THE DIGITAL-TO-ANALOG CONVERTER (DAC)
The DAC sets the threshold voltage for chopping at V
DAC REF
× 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 9b and Figure 9e again). A parallel resistor-capacitor network connected 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
× D/16) ÷ ((250 × 10
−6
) × R
S
)) amperes. (2)
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 × 15/16) ÷ ((250 × 10
−6
) × 6.25 kΩ)) = 3.00A (3)
The resulting typical DAC programmable current limit values, for different values of R
S
, would be:
Table 1. D to A winding current thresholds for V
REF DAC
= 5.00V
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
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