BU64985GWZ Datasheet, PDF (8/25 Pages) Rohm – Bi-directional VCM driver for Auto focus

BU64985GWZ
Additionally, based on the REF value, the minimum output current of the BU64985GWZ is calculated as:
𝐼𝑜_{𝑚𝑖𝑛}
=
( ¹²⁰ )
1023
∗
−𝑅𝐸𝐹
[mA]
Equation 2. Minimum Output Current Calculation
Figure 8. Example of Reference Code Setting
Please note that when calculating a REF value based on a target output current, the resulting REF value needs to be
converted to an 8-bit DAC code by removing the 2 LSBs.
The reference DAC should be set based on the properties of the VCM actuator. A traditional VCM actuator uses a barrel
that rests against the mechanical end of the actuator when no current is applied to the coil. Using a traditional VCM
requires the reference DAC to be set to 0. Bidirectional VCM actuators have the natural position set based on the
actuator manufacturers’ process. Typically the reference DAC for bidirectional VCM actuators should be set so that the
output current range matches the natural full stroke range as closely as possible. The reference DAC is set to 0x80 as
a default after power initialization.
Controlling Mechanical Ringing
A 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
BU64985GWZ 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 illustrates how to best utilize ISRC to minimize mechanical oscillations.
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 has 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 TDAC[9:0] value in register W[2:0] = 0b010 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 TDAC[9:0], the EN bit must be set to 1.
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6. Oct.2016 Rev.003