BU64987GWZ Datasheet, PDF (12/25 Pages) Rohm – Bi-directional VCM driver for Auto focus

BU64987GWZ
Displacement
(µm)
Bidirectional VCM
Naturally
floating
Traditional VCM
0
Sequence start point
Time
(ms)
Figure 13. Three Mode Sequential Operation (Shown as Lens Displacement)
Bidirectional VCM actuators require a non-zero reference DAC to be set. If the reference DAC is set to any value other
than 0, then the ISRC behavior will ignore point A, point B, and step mode settings and instead only use the point C final
target DAC position due to the bidirectional VCM already existing in a floating state.
Controlling the ISRC Settling Time
The settling time of an actuator is the time it takes for ringing to cease. The BU64987GWZ is able to control the settling
time by modifying the slew rate speed parameter, however care must be taken to balance settling time vs. acceptable
ringing levels. By increasing the slew rate speed there is the possibility to decrease the settling time but the ability to
control ringing is also decreased. Likewise, if less ringing is desired then there is a possibility to reduce the ringing levels
by using a slower slew rate speed setting at the cost of longer settling times. The slew rate speed can be set by modifying
the 2-bit slew_rate[1:0] value located at the 2 LSBs of register W[2:0]=0b100. Figure 4 shows the relationship of slew
rate speed vs. settling time.
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slew_rate(1:0)=2⁰'^bb¹1¹1
slew_rate(1:0)=20'b10
slew_rate(1:0)=20'bb0011
slew_rate(1:0)=20'bb0000
0
Target DAC update
Time
(ms)
Figure 14. Slew Rate Speed vs. Settling Time
Table 7. Slew Rate Speed Settings
slew_rate[1:0]
Slew Rate
Speed
slew_rate[1:0]
0b00
Slowest
0b01
Slew Rate
Speed
Slow
slew_rate[1:0]
0b10
Slew Rate
Speed
Fast
slew_rate[1:0]
0b11
Slew Rate
Speed
Fastest
DAC Code Update Timing Considerations
Settling time is controlled by the resonant frequency of the actuator and the driver’s slew rate speed setting. Depending
on the combination of these parameters, the settling time can be such that updating point C with a new DAC code before
the lens has settled at the original point C DAC code can adversely affect the settling time due to increased ringing effects.
Utilize the slew rate speed parameter in order to modify the settling time so that any updates to the point C DAC code do
not occur before the lens has settled.
Please review the following example based on an actuator with a resonant frequency of 100 Hz:
Table 8. Relationship Between Slew Rate Speed and Settling Time Based on a 100Hz Actuator
Resonance Frequency f0 (Hz) slew_rate[1:0] Settling Time (ms)
0b00
40
0b01
24
100
0b10
16
0b11
12
In this example the settling time of the actuator can vary by up to ±5% due to the internal oscillator (MCLK) having a
variance of ±5%. The settling time has a proportionally inverse relationship to the resonant frequency and therefore the
settling time can be estimated as:
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TSZ02201-0K1K0B601860-1-2
6. Oct.2016 Rev.001