AD5821 datasheet(13/16 Pages) AD

13 / 16 page

AD5821

Rev. 0 | Page 13 of 16

When sinking the maximum current of 120 mA, the maximum

voltage drop allowed across RSENSE is 400 mV, and the minimum

drain to source voltage of Q1 is 200 mV. This means that the

AD5821 output has a compliance voltage of 600 mV. If VDROP

falls below 600 mV, the output transistor, Q1, can no longer

operate properly and ISINK may not be maintained as a constant.

When sinking 90 mA, the maximum voltage drop allowed

across RSENSE is 300 mV, and the minimum drain to source

voltage of Q1 is 180 mV. This means that the AD5821 output

has a compliance voltage of 480 mV. If VDROP falls below 480 mV,

the output transistor, Q1, can no longer operate properly and

ISINK may not be maintained as a constant. As ISINK decreases, the

voltage required across the transistor, Q1, also decreases and,

therefore, lower supplies can be used with the voice coil motor.

As the current increases to 120 mA through the voice coil,

VC increases. VDROP decreases and eventually approaches the

minimum specified compliance voltage of 600 mV (or 480 mV,

if ISINK = 90 mA). The ground return path is modeled by the

components RG and LG. The track resistance between the voice

coil and the AD5821 is modeled 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, may 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, if

VBATTERY = 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

DROP

BAT

)]

(

)

(

[

120

Ω)]

0.5

(120

[600

3.6

−

Using another example, if

VBATTERY = 3.6 V

RG = 0.5 Ω

RT = 0.5 Ω

ISINK = 90 mA

VDROP = 480 mV (the compliance voltage specification at 90 mA)

Then the largest value of resistance of the voice coil, RC, is

−

SINK

DROP

BAT

)]

(

)

(

[

33.66

Ω)]

0.5

(90

[480

3.6

−

For this reason, it is important to minimize any series impedance

on both the ground return path and interconnect between the

AD5821 and the motor. It is also important to note that for

lower values of ISINK, the compliance voltage of the output stage

also decreases. This decrease allows the user to either use voice

coil motors with high resistance values or decrease the power

supply voltage on the voice coil motor. The compliance voltage

decreases as the ISINK current decreases.

The power supply of the AD5821, or the regulator used to supply

the AD5821, should be decoupled. Best practice power supply

decoupling recommends that the power supply be decoupled

with a 10 μF capacitor. Ideally, this 10 μF capacitor should be of

a tantalum bead type. However, if the power supply or regulator

supply is well regulated and clean, such decoupling may not be

required. The AD5821 should be decoupled locally with a 0.1 μF

ceramic capacitor, and this 0.1 μF capacitor should be located as

close as possible to the VDD pin. The 0.1 μF capacitor should be

ceramic 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, they should run at

right angles to each other to reduce feedthrough effects through

the board. The best 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.

AD5821 bảng dữ liệu(PDF) 13 Page - Analog Devices

AD5821 bảng dữ liệu(HTML) 13 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

bảng dữ liệu tải về

Link URL