BU64985GWZ Datasheet, PDF (10/25 Pages) Rohm – Bi-directional VCM driver for Auto focus

BU64985GWZ
Figure 10 illustrates the different operational modes that control the autofocus algorithm. The green line represents the
ideal operation sequence of a bidirectional VCM actuator and the red line represents the ideal operation sequence of a
conventional VCM actuator. Due to ISRC requiring a floating lens, a traditional VCM actuator (non-bidirectional) requires
points A and B to be set in order to create a floating condition. In order to simplify the code sequence, it is possible to
skip setting point A and instead only set point B, however if an optimized ringing control method is preferred, point A
corresponds to the maximum amount of current that can be applied to all VCM units without floating the lens. Point B
corresponds to the minimum amount of current that can be applied to the VCM so that all actuator units are floating. It
should be noted that the target DAC codes could vary between different actuator units and that sufficient evaluation should
be performed before selecting the point A and B target DAC codes. Point C is the final lens target position determined
by the level of focus required for the image capture and bidirectional VCM actuators only require point C for proper ISRC
operation.
The actuator manufacturer should be able to provide the required current for points A and B, however it is possible to test
these points by slowly increasing the 10-bit value of TDAC[9:0] and measuring the lens movement using a laser
displacement meter or some other device to measure lens displacement.
Output Current Control
After characterizing the VCM performance, the following should be performed in order to properly control the driver
settings for optimized autofocus performance.
Setting Point A, B, and C DAC Codes
Points A, B, and C are defined by 10-bit DAC codes set with the following registers:
Table 4. Target DAC Code Register Locations
Location
W[2:0] Register
DAC Code Location
Point C
0b010
TDAC[9:0]
Point A
0b101
ADAC[9:0]
Point B
0b110
BDAC[9:0]
Description
Final lens position before image capture
Maximum output current without floating the lens
Minimum output current required to float the lens
Please note that when the reference DAC is set to a non-zero value, due to use with a bidirectional VCM actuator, points A
and B are ignored and only point C is used for target DAC positions.
Controlling Direct Mode
Direct mode is when the driver outputs the desired amount of output current with no output current control. The time in
which the lens reaches the position that corresponds to the amount of output current set by the 10-bit DAC code is ideally
instant, ignoring the ringing effects. If the driver is set so that the lens is moved from a resting position to point C with
direct mode, ringing and settling time will be at a maximum.
Direct mode is used either when M=0 or when M=1, the reference DAC is set to 0, and the present DAC code is less than
the DAC code of point A.
M = 0 = ISRC mode disabled
When ISRC mode is disabled by setting the M bit equal to 0, the lens will traverse to the DAC code set for point C when
the EN bit is set equal to 1.
M = 1 = ISRC mode enabled
The driver automatically uses direct mode if the present DAC code is less than the target DAC code corresponding to
point A. Therefore during ISRC operation when the autofocus sequence has been started by setting the EN bit equal to
1, the driver will automatically decide to use direct mode to output current up to point A and then switch to step mode
before continuing the autofocus sequence.
10-bit target DAC is updated and
movement starts at 3^rd ACK
Time (s)
Figure 11. Direct Mode Output Current vs. Time
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6. Oct.2016 Rev.003