AD5398
Rev. B | Page 13 of 16
As the current increases through the voice coil, VC increases
and VDROP decreases and eventually approaches the minimum
specified compliance voltage of 600 mV. The ground return
path is modelled by the components RG and LG. The track
resistance between the voice coil and the AD5398 is modelled as
RT. The inductive effects of LG influence RSENSE and RC equally,
and because the current is maintained as a constant, it is not as
critical as the purely resistive component of the ground return
path. When the maximum sink current is flowing through the
motor, the resistive elements, RT and RG, might have an impact
on the voltage headroom of Q1 and could, in turn, limit the
maximum value of RC because of voltage compliance.
For example:
VBAT = 3.6 V
RG = 0.5 Ω
RT = 0.5 Ω
ISINK = 120 mA
VDROP = 600 mV (the compliance voltage)
Then the largest value of resistance of the voice coil, RC, is
=
×
+
×
+
=
SINK
G
SINK
T
SINK
DROP
BAT
C
I
R
I
R
I
V
R
)]
(
)
(
[
Ω
24
mA
120
Ω)]
0.5
mA
(120
2
mV
[600
V
3.6
=
×
+
For this reason it is important to minimize any series impedance
on both the ground return path and interconnect between the
AD5398 and the motor.
The power supply of the AD5398 should be decoupled with
0.1 μF and 10 μF capacitors. These capacitors should be kept as
physically close as possible, with the 0.1 μF capacitor serving as
a local bypass capacitor, and therefore should be located as close
as possible to the VDD pin. The 10 μF capacitor should be a
tantalum bead-type; the 0.1 μF capacitor should be a ceramic
type with a low effective series resistance and effective series
inductance. The 0.1 μF capacitor provides a low impedance path
to ground for high transient currents.
The power supply line should have as large a trace as possible to
provide a low impedance path and reduce glitch effects on the
supply line. Clocks and other fast switching digital signals
should be shielded from other parts of the board by digital
ground. Avoid crossover of digital and analog signals if possible.
When traces cross on opposite sides of the board, ensure that
they run at right angles to each other to reduce feedthrough
effects through the board. The best board layout technique is to
use a multilayer board with ground and power planes, where
the component side of the board is dedicated to the ground
plane only and the signal traces are placed on the solder side.
However, this is not always possible with a 2-layer board.
The exposed paddle on the AD5398 should be soldered to
ground to ensure the best possible thermal performance. The
thermal impedance of the AD5398 LFCSP package is 48°C/W
when soldered in a 4-layer board. It is defined in the
Absolute