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Power Drive Circuits 295048-AN
Power Drive Circuits
Power switching circuitry is required to complete
the interface for electrical control signals between
microcontrollers and system loads. The real-world
function of a load may be in the form of motion,
light, or sound. Depending on the complexity of
the control system, the interface circuit may be
required to control a simple action, to provide
feedback signals, or to perform fault isolation.
To serve a wide range of requirements, Allegro
®

offers a complete portfolio of power interface
circuitry. Manufactured using the Allegro
ABCD3 process technology, the product line
offers designers flexibility in both architecture
and power performance, enabling a variety of
solutions for individual applications.
This new family of Allegro power ICs are power
DMOS devices that feature multiple integrated
transistors in surface-mount and DIP packaging.
The devices include on-chip control logic, ESD
protection, and clamping circuitry.
Due to the high level of integration, Allegro
power ICs maintain significant cost advantages
over discretes on a per-transistor basis. These
advantages include reduced component count and
board-space requirements, as well as minimized
procurement and inventory expenses.
By the Integrated Circuits Business Unit
Application Information
Introduction
Power Applications
...used in a wide variety of end equipment:
EDP
Industrial
Automotive
HDDs
Automated test equipment
Powertrain
Tape back-ups
Process control systems
Body and chassis
Printers
Programmable machine tools
Instrumentation
Plotters
Robotics
Passive restraints
Copiers
Instrumentation panels
ABS
Scanners
Personal appliances
EFE
Fax machines
Telecom line cards
Electronic games 295048-AN
2
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Allegro power interface devices offer superior alter-
natives to discrete power MOSFETs and hybrids in
many power switching applications, including: driv-
ing fractional horsepower motors, solenoids, valves,
relays, and lamps.
In the market today, significant application areas
include:
EDP: hard-disk drives, tape back-ups, printers, plot-
ters, copiers, scanners, fax machines, and power
distribution switching
With their low on-resistance and minimized power
dissipation, these power devices operate reliably in
confined spaces. Their surface-mount packaging is
well-suited for modules with limited headroom.
Industrial: automated test equipment, process con-
trol systems, programmable machine tools, robotics,
instrumentation panels, personal appliances, tele-
com line cards, moving signs, and electronic games
The ruggedness of these devices makes them very
attractive for industrial environments. They offer
power handling capabilities, extended temperature
ranges, and avalanche energy absorption.
Automotive: powertrain (engine, transmission, and
emission controls), body and chassis, instrumenta-
tion, passive restraints, anti-lock brake systems, and
electronic fuel injectors
For this cost-conscious segment and its extremely
harsh operating environment, these devices offer
wide operating ranges, a high level of integration,
short cycle time to market, and cost-effectiveness.
These are low-cost and low-risk catalog alternatives
to custom solutions.
Types of Loads
There are several types of loads, such as: resistive,
capacitive, and inductive. Resistive loads are the sim-
plest, because sizing is largely a question of examin-
ing the current and voltage specifications, estimating
dissipation, making allowance for duty cycle, and then
allowing margins for safety. Capacitive loads are com-
paratively rare. Stray capacitance is the only capaci-
tive element in an otherwise resistive load.
Inductive loads, however, are relatively common
and can be complex to design because energy can be
passed from the switch to the load and back again to
the switch. This energy must be dissipated without
damaging the load or switch. A well-specified ava-
lanche energy value for the switch is helpful.
Inductive Load Switch Requirements
Motors, solenoids, lamps, and other assorted loads are
generally specified by operating voltage and current.
The information provided in manufacturer datasheets
is sufficient for operating at continuous duty. How-
ever, in most applications, the load is being switched
on and off. When switching loads, the operating
requirements as well as transient conditions must be
considered. The power requirements are often further
influenced by dynamic operating conditions.
The easiest way to look at load requirements is to
consider the example of a load operating from a bat-
tery and controlled by a low-side switch. The system
power supply and load choice will determine:
Current drawn from the battery, including transients
when the switch is turned on and off
Battery terminal voltage
Energy output from the load (motion, sound, etc.)
Energy dissipated from the load in the form of heat
(I
2
R loss, magnetic loss, and friction)
Energy returned to the system (induction, regenera-
tion, and cross-coupling)
These system load requirements must then be used to
determine the switch requirements:
Continuous drain-source current
Pulsed drain current
Continuous power dissipation at T
A
= 25°C
Single-pulse avalanche energy (energy returned to
the switch from back EMF)
Drain-source voltage, V
DS

Drain-source on-state resistance, R
DS(on)
295048-AN
3
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
+I
+I
+I
t
on
t
off
Incandescent Lamp
Solenoid
t
r
In-rush Current
Armature
Movement
I
LOAD
I
LOAD
I
LOAD
t
t
t
Stepper Motor
A
B
C
Load switching current characteristics for three common power applications
Selecting or designing a switch is a three-step process:
1. Determine the total energy, current, and voltage
required.
2. Select a switching device that will accommodate the
energy.
3. Evaluate the system power dissipation to determine
any heat-sinking requirements.
Load Switching Current Levels
Determining the total energy begins with evaluating
the load current, both during operation and during
switching. The diagram below shows the load current
waveforms for an incandescent lamp, a solenoid, and
a stepper motor. Each is depicted at steady state and at
switching conditions such as those that must be con-
sidered in controlling a load. 295048-AN
4
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
The incandescent lamp current shows a high in-rush
value at initial turn-on (A), due to the difference in
filament resistance when cold and hot. During the on
period, t
on
, the current level decreases to a steady cur-
rent value until turn-off (B). There is a clean turn-off
,with no current flowing during the off period, t
off
. A
lamp control switch will need to withstand high peak
currents or limit the current until the lamp filament
warms up. The latter approach is preferable, because it
extends the life of the filament.
The solenoid current starts rising at turn-on (A) and
increases until turn-off (B), but after turn-off contin-
ues to flow during a period, t
r
, until it dissipates fully
(C). The change in current slope between turn-on and
turn-off is caused by the solenoid armature moving
closer to the coil and increasing the coil inductance.
The current flow during t
r
is a result of the magnetic
field in the solenoid collapsing and returning energy
to the system. A solenoid switch must be capable of
conducting the coil operating current, and the system
must provide a method of accommodating the energy
returned to the system at turn-off. Several methods
are employed to deal with the returned energy, which,
when it is dissipated in the switch, is referred to as
avalanche energy.
The stepper motor current waveform exhibits the
exponential increase characteristic of an inductive
load. Returned energy is a factor in stepper motor con-
trol. Additionally, stepper motor windings can produce
currents as a result of cross-coupling from adjacent
motor windings. This is particularly true for unipolar
stepper motors. A control circuit for a stepper motor
must accommodate the transient energy at turn-on (A)
and the returned energy at turn-off (B).
When considering a switch for a stepper motor