BU64241GWZ Datasheet, PDF (11/24 Pages) Rohm – Linear Constant Current VCM Driver

BU64241GWZ
Datasheet
●Controlling Mechanical Ringing
A voice coil motor (VCM) is an actuator technology that is intrinsically noisy due to the properties of the mechanical
spring behavior. As current passes through the VCM, the lens moves and oscillates until the system reaches a steady
state. The BU64241GWZ lens driver is able to control mechanical oscillations by using the integrated ISRC (intelligent
slew rate control) function. ISRC is operated by setting multiple control parameters that are determined by the intrinsic
characteristics of the VCM. The following steps illustrate how to best utilize ISRC to minimize mechanical oscillations.
・Step A1 – Determining the Resonant Frequency of the VCM
Each VCM has a resonant frequency that can either be provided by the manufacturer or measured. The resonant
frequency of an actuator determines the amount of ringing (mechanical oscillation) experienced after the lens as been
moved to a target position and the driver output current held constant. To determine the resonant frequency, f₀, input a
target DAC code by modifying the 10 bit C_DAC[9:0] value in register W2W1W0 = 000b that will target a final lens
position approximately half of the actuator’s full stroke. Take care to not apply too much current so that the lens does
not hit the mechanical end of the actuator as this will show an incorrect resonant period. In order to start movement of
the lens to the DAC code that was set in C_DAC[9:0], the EN bit must be set to 1.
T
???
? µm?
0
Time (mTism)e
(ms)
Figure 14. Actuator Displacement Waveform (ISRC Disabled)
The resonant frequency (Hz) of the actuator can be calculated with Equation 1 using the resonant period observed in
Figure 14.
f₀ = (T)^-1
Equation1. Resonant Frequency vs. Time Period Relationship
After calculating the correct resonant frequency, program the closest value in the W2W1W0 = 001b register using the 5
bit rf[4:0] values from Table 1. When calculating the resonant frequency take care that different actuator samples’
resonant frequencies might vary slightly and that the frequency tolerance should be taken into consideration when
selecting the correct driver resonant frequency value.
Table 1. f₀ Settings (rf[4:0])
rf[4:0]
f₀
rf[4:0]
f₀
rf[4:0]
f₀
rf[4:0]
f₀
00000
-
01000
85 Hz
10000
125 Hz
11000
-
00001
50 Hz
01001
90 Hz
10001
130 Hz
11001
-
00010
55 Hz
01010
95 Hz
10010
135 Hz
11010
-
00011
60 Hz
01011
100 Hz
10011
140 Hz
11011
-
00100
65 Hz
01100
105 Hz
10100
145 Hz
11100
-
00101
70 Hz
01101
110 Hz
10101
150 Hz
11101
-
00110
75 Hz
01110
115 Hz
10110
-
11110
-
00111
80 Hz
01111
120 Hz
10111
-
11111
-
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TSZ22111・15・001
11/20
TSZ02201-0H2H0B600460-1-2
01. Jul. 2013 Rev.004