BU64987GWZ Datasheet, PDF (11/25 Pages) Rohm – Bi-directional VCM driver for Auto focus

BU64987GWZ
Controlling Step Mode
Step mode is the control period in which the lens is moved by small output current steps. During step mode it is possible
to control the step resolution and step time in order to generate just enough output current to float the lens with minimal
ringing effects. Ringing can be better controlled by choosing a large value for the step time and a small value for the
step resolution with the trade off of a greater settling time. The step time and step resolution should be chosen depending
on the acceptable system limits of ringing vs. settling time.
Step mode is used when M=1, the reference DAC set to 0, and the present DAC code is in between point A and point B.
Typically this mode is only used during ISRC operation between point A and B, however it is possible to move the lens to
point C using only step mode if point C is set such that point C is only 1 DAC code greater than point B.
Step mode is controlled by the 5-bit step time, stt[4:0], and 3-bit step resolution, str[2:0], values stored in register W[2:0]
= 0b111. The step time is set by the 5 LSBs and the step resolution is set by the 3 MSBs of the third byte write while
using register W[2:0] = 0b111.
Table 5. Step Time Settings
stt[4:0] Step Time stt[4:0] Step Time stt[4:0] Step Time stt[4:0] Step Time
0b00000
-
0b01000
400 µs
0b10000
800 µs
0b11000 1200 µs
0b00001
50 µs
0b01001
450 µs
0b10001
850 µs
0b11001 1250 µs
0b00010
100 µs
0b01010
500 µs
0b10010
900 µs
0b11010 1300 µs
0b00011
150 µs
0b01011
550 µs
0b10011
950 µs
0b11011
1350 µs
0b00100
200 µs
0b01100
600 µs
0b10100
1000 µs
0b11100
1400 µs
0b00101
250 µs
0b01101
650 µs
0b10101
1050 µs
0b11101
1450 µs
0b00110
300 µs
0b01110
700 µs
0b10110
1100 µs
0b11110
1500 µs
0b00111
350 µs
0b01111
750 µs
0b10111
1150 µs
0b11111
1550 µs
Table 6. Step Resolution Settings
str[2:0]
Step
Resolution
str[2:0]
0b000
0b001
-
1 LSB
0b010
0b011
Step
Resolution
2 LSB
3 LSB
str[2:0]
0b100
0b101
Step
Resolution
4 LSB
5 LSB
str[2:0]
0b110
0b111
Step
Resolution
6 LSB
7 LSB
The BU64987GWZ has an absolute output current range of 120mA which corresponds to a step resolution of
0.117mA/LSB.
Using a normal VCM actuator (non-bidirectional), it is possible to skip step mode during ISRC operation if a simpler
autofocus code sequence is desired. If there is no issue with moving the lens to point B using direct mode, then the
DAC code for point A should be left equal to 0. Additionally if the point A register is not set after the driver is initialized,
then the driver will automatically move the lens to point B with direct mode since the default value for point A is 0.
Controlling ISRC Mode
ISRC mode is the control period in which the lens is already floating and the driver smoothly moves the lens based on
the proprietary behavior of the ISRC algorithm. ISRC operation keeps ringing at a minimum while achieving the fastest
possible settling time based on the ISRC operational conditions.
ISRC mode is used when M=1, the reference DAC set to 0, and the present DAC code is greater than the DAC code for
point B. ISRC mode is also used when M=1 and the reference DAC set to a non-zero value. If the target DAC code
for point C is set so that the value is too large and will cause excess ringing, the point C DAC code is automatically
updated with a driver pre-determined value to minimize the ringing effect. When M=1 and the reference DAC set to 0,
the driver will automatically switch between direct mode, step mode, and ISRC mode when the point A, B, and C DAC
code conditions are met. The condition for this automatic transitioning to occur is when the register values for point B
and point C are set to values other than 0 and then the sequence will start when the EN bit is set equal to 1. Please
note that updates to point B and C DAC codes should be avoided during a focus operation in order to minimize poor
ringing effects.
DAC code
C
B
ISRC DAC codes*
A
Step mode
Direct mode
0
Start sequence
ISRC mode _{※ ISRC DAC codes}
– the details of ISRC
operation are
proprietary
Time (ms)
Figure 12. Three Mode Sequential Operation (Shown as DAC Codes) for Traditional VCM Actuators
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TSZ22111・15・001
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TSZ02201-0K1K0B601860-1-2
6. Oct.2016 Rev.001