Full-Bridge DMOS PWM Motor Driver

A4950

7

Allegro MicroSystems, Inc.

115 Northeast Cutoff

Worcester, Massachusetts 01615-0036 U.S.A.

1.508.853.5000; www.allegromicro.com

Application Information

Sense Pin (LSS)

In order to use PWM current control, a low-value resistor is

placed between the LSS pin and ground for current sensing pur-

poses. To minimize ground-trace IR drops in sensing the output

current level, the current sensing resistor should have an indepen-

dent ground return to the star ground point. This trace should be

as short as possible. For low-value sense resistors, the IR drops in

the PCB can be significant, and should be taken into account.

When selecting a value for the sense resistor be sure not to

exceed the maximum voltage on the LSS pin of ±500 mV at

maximum load. During overcurrent events, this rating may be

exceeded for short durations.

Ground

A star ground should be located as close to the A4950 as possible.

The copper ground plane directly under the exposed thermal pad

of the device makes a good location for the star ground point. The

exposed pad can be connected to ground for this purpose.

Layout

The PCB should have a thick ground plane. For optimum

electrical and thermal performance, the A4950 must be soldered

directly onto the board. On the underside of the A4950 package is

an exposed pad, which provides a path for enhanced thermal dis-

sipation. The thermal pad must be soldered directly to an exposed

surface on the PCB in order to achieve optimal thermal conduc-

tion. Thermal vias are used to transfer heat to other layers of the

PCB.

The load supply pin, VBB, should be decoupled with an electro-

lytic capacitor (typically 100 μF) in parallel with a lower valued

ceramic capacitor placed as close as practicable to the device.

GND

RS

GND

GND

GND

C1

BULK

CAPACITANCE

VBB

OUT2

OUT1

C2

1

PAD

A4950

C1

C2

OUT2

LSS

OUT1

VBB

IN2

IN1

VREF

GND

PCB

Thermal Vias

Trace (2 oz.)

Signal (1 oz.)

Ground (1 oz.)

Thermal (2 oz.)

Solder

A4950

Bill of Materials

Item

Reference

Value

Units

Description

1RS

0.25

Ω

2512, 1 W, 1% or better,

carbon film chip resistor

2

C1

0.22

μF

X5R minimum, 50 V or greater

3

C2

100

μF

Electrolytic, 50 V or greater