Low Voltage Voice Coil Motor Driver
A3904
6
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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 .
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
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.
Application Information
Figure 1. Relative accuracy error Figure 2. Gain error
Relative Accuracy ( Codes 16-255), errors exaggerated for clarity
Code Code
I
OUT
(mA)
I
OUT
(mA)
Straight line between measured Codes 16 and 255
Actual DAC, errors exaggerated for clarity
Calculated Gain Error, Offset Error removed
0
016 16128 128255 2550
32.0
64.0
96.0
127.5
0
127.5
Ideal DAC
Offset
Error
Gain
Error
INL
