A3907 データシートの表示（PDF） - Allegro MicroSystems
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A3907 Datasheet PDF : 11 Pages
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A3907
Low Voltage Voice Coil Motor Driver
Functional Description
Output Current Level Control
The A3907 output current level, IOUT , is controlled dynamically
by programming the D-to-A converter (DAC) value via the I2C
serial port. A 10-bit Level Control code, having a decimal equiva-
lent value from 0 through 1023, is clocked into the SDA pin.
The target output current can be calculated by:
IOUT = nDAC × 100 μA ,
(1)
where nDAC is the decimal equivalent of the Level Control code.
For example, a code of 5 (000001012) sets an output current
target of 500 μA.
Programming Level Control code 0 disables the output sink
drive. In addition, the DAC is automatically set to code 0 at
power-up and also at a fault condition on VDD.
Output Current Slew Rate Control
When a new current level control instruction is received on the
SDA input, the A3907 moves to the new target currrent level by
incrementing or decrementing through each of the intermediate
current levels until it arrives at the new programmed value.
The control instruction received at the SDA input includes both
the 10-bit Level Control code and a 4-bit Ramp Control code.
The Level Control code is used to determine the absolute value of
the changes in IOUT (see equation 1), and the Ramp Control code
maps to a lookup table of time intervals (represented in table 1).
Together, these two codes determine the shape of the current level
change function.
Step or Ramp Function The A3907 can change to the new
target level using either a step or a ramp slew rate function. When
a step function is selected, the A3907 moves to the new target
level without imposing any additional time delays between DAC
updates. To select a step function, program one of the four Ramp
Control codes in table 1 that disable the ramp feature.
When a ramp function is selected, the A3907 imposes time delays
between each DAC update, calculated according to the particular
function option selected. To select a ramp function, program one
of the twelve Ramp Control codes in table 1 that enable the ramp
feature.
Single or Dual Subintervals For either the step or the ramp
slew rate method, the total change can be accomplished in either
one continuous time interval, or divided over two sequential time
subintervals. When the single-interval method is selected, the total
change in IOUT is accomplished over the total time interval deter-
mined by the Rate Control code in table 1, calculated as follows:
CodeT = |(CodeNewTarget – CodePreviousTarget)| / 2 ,
(2)
and
T = CodeT × tdT ,
(3)
where tdT is the delay factor, in table 1.
When the dual-subintervals method is selected, the elapsed time
for each subinterval is determined separately by the Rate Con-
trol code in table 1. The time interval from initiation, T0, to the
switchover point, T1, is calculated as follows:
CodeSwitchover = |(CodeNewTarget – CodePreviousTarget)| / 2 , (4)
and
T1 = CodeSwitchover × tdT1 ,
(5)
where tdT1 is the delay factor for the initial time subinterval, in
table 1.
The current amplitude at the switchover point is calculated based
on equation 1, as follows:
ISwitchover = (CodeLow + CodeSwitchover ) × 100 μA ,
(6)
where CodeLow is the lesser of CodeNewTarget and CodePreviousTarget .
The time interval from the switchover point, T1, until the target
current level is reached, T2, is calculated as follows:
T2–T1 = CodeSwitchover × tdT2 ,
(7)
where tdT2 is the delay factor for the second time subinterval, in
table 1.
Output Function Programming
Two examples of output level and slew rate programming are
shown in figure 1. Both examples are ramp slew rate functions,
using the dual-subinterval method. In example A, an increment in
IOUT is shown, and example B shows a decrement in IOUT.
Allegro MicroSystems, LLC
5
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
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