A3904
Low Voltage Voice Coil Motor Driver
Application Information
Headroom
The current may not reach the programmed level
if there is not adequate headroom in the output circuit. The IC
output voltage must be over 500 mV to guarantee normal linear
operation. V
DD
, I
LOAD
, and R
LOAD
can be adjusted to ensure that
the device operates in the linear range.
If the equation shown below is not satisfied, the load current
will be limited by the series impedance, and may not reach the
programmed level
V
DD
(min) – R
LOAD
(max) × I
OUT
(max)
≥
500 mV .
be positive for each incremental step, according to the following
formula:
DNL = (I
OUT(n+1)
–
I
OUT(n)
) / LSB – 1 (LSB) .
where
n
is in the range 16 to 255.
Offset error
The measured output current at input Code = 16,
compared to the ideal value according to the transfer function
(8 mA).
Gain Error
The difference in the slopes of the ideal transfer
function and the actual transfer function. The gain error is calcu-
lated by subtracting out the offset error, at Code = 16, from the
actual transfer function. This calculated value is compared to the
ideal transfer function and reported as a percentage of the ideal
full scale value (127.5 mA). See figure 2.
Gain Error Drift
The change in slope of the transfer function
due to temperature, expressed as LSB/°C.
I
OUT
Errors
Relative accuracy (INL)
This error is calculated by measuring
the worse case deviation from a straight line, defined from end
points. The straight line end points are defined by the actual mea-
sured values at Code = 16 and Code = 255. See figure 1.
Differential nonlinearity (DNL)
A measure of the monotonic-
ity of the D-to-A converter. The slope of the line must always
127.5
INL
96.0
I
OUT
(mA)
127.5
I
OUT
(mA)
Offset
Error
Gain
Error
64.0
32.0
0
0
16
128
Code
Straight line between measured Codes 16 and 255
Relative Accuracy ( Codes 16-255), errors exaggerated for clarity
255
0
0
16
128
Code
Ideal DAC
Actual DAC, errors exaggerated for clarity
Calculated Gain Error, Offset Error removed
255
Figure 1. Relative accuracy error
Figure 2. Gain error
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
6
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
