EVAL-AD5398EB【PDF 12/16 页】120 mA, Current Sinking, 10-Bit, I2C DAC

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AD5398

As the current increases through the voice coil, V

increases

and V

DROP

decreases and eventually approaches the minimum

specified compliance voltage of 600 mV. The ground return

path is modeled by the components R

and L

, and the track

resistance between the voice coil and the AD5398 is modeled as

. The inductive effects of L

influence R

SENSE

and R

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, R

and R

, might have an impact

on the voltage headroom of Q1 and could, in turn, limit the

maximum value of R

because of voltage compliance.

Rev. A | Page 12 of 16

For example,

BAT

= 3.6 V

= 0.5 Ω

SINK

= 120 mA

DROP

= 600 mV (the compliance voltage)

Then the largest value of resistance of the voice coil, R

, is

SINK

DROP

[

BAT

)]

(

)

(

120

Ω)]

0.5

(120

[600

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 V

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 itself 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

Maximum Ratings Section.

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