itches allow
current to flow bidirectionally, the voltage across the load
and the direction of current through the load can be of either
polarity.
H-Bridges are often used to control the speed, position or
torque of DC and stepper motors. Traditionally implemented
with either discrete or monolithic bipolar transistors, fully
integrated solutions are becoming increasingly popular in
printer, plotter, robotics and process control applications that
require 0.5A to 3.0A and operate from 12V to 55V. The
LMD18200 was designed to operate within this range and
was optimized for such applications.
The LMD18200 was implemented in a process that allows
bipolar, CMOS and DMOS devices to be incorporated to-
gether on one die. As each of these types of transistor
structures has its own unique characteristics, each is ideally
suited for a different function. By integrating them together,
this allowed us to take advantage of several innovative
design techniques to provide easy to use benefits typically
unassociated with a simple motor driver.
Figure 2 shows a functional block diagram of the LMD18200.
The circuit contains four DMOS power switching transistors,
with intrinsic clamp diodes, connected in an H-Bridge con-
figuration. All level shifting and drive circuits are included to
permit control of the H-Bridge from standard logic compat-
ible signal levels. Other unique features include current
sense circuitry, overcurrent and under-voltage protection,
thermal warning and thermal shutdown. Each is discussed in
more detail in the following section.
Key Features
DMOS POWER DRIVERS
DMOS power transistors allow current to flow bidirectionally
and provide a lower voltage drop than similarly rated bipolar
power transistors by virtue of a greatly reduced on resis-
tance for each switch. They also have the potential to oper-
ate at much faster switching speeds for more efficient opera-
tion. And, as each switch contains its own intrinsic protection
diode, the additional external protection diodes that are re-
quired for bipolar transistor implementations are no longer
necessary.
LOW ON RESISTANCE
Unlike bipolar transistors, which have a relatively high volt-
age drop across them, even at lower currents, the DMOS
devices in the LMD18200 have a voltage drop that is essen-
tially a linear function of temperature. The on resistance,
R
DS(on)
, of each output transistor is typically 0.3
at a junc-
tion temperature of 25C and 0.6
at 125C. At 100C and
1A of current, a comparable bipolar transistor will have a
voltage drop from collector to emitter of about 1.1V whereas
with the LMD18200 this voltage drop will only be 0.45V. At
higher current levels the lower voltage drop across a DMOS
power device provides an appreciable reduction in power
dissipation resulting in smaller heat sink requirements and
better efficiency with more power throughput to the load.
01085901
FIGURE 1. Basic H-Bridge Circuit
National Semiconductor
Application Note 694
Tim Regan
December 1999
A
DMOS
3A,
55V
,
H-Bridge:
The
LMD18200
AN-694
© 2002 National Semiconductor Corporation
AN010859
www.national.com Key Features
(Continued)
01085902
FIGURE 2. Block Diagram of the LMD18200
01085903
FIGURE 3. A DMOS Switch with Intrinsic Protection Diode
01085904
FIGURE 4. Waveforms Illustrating the Commutation of Reverse Current
in One Switch (A1) to Forward Current in Another Switch (A2)
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2 Key Features
(Continued)
BIDIRECTIONAL CURRENT SWITCHES WITH
INTRINSIC PROTECTION DIODES
When driving inductive and inertial loads such as motors the
power switches must be able to conduct forward as well as
reverse current. The energy stored in these types of loads
must generally be free to return to the supply.
The conventional method of providing a path for reverse
current is to connect an antiparallel diode across the power
switch as shown in Figure 3.
With the DMOS structure used in the LMD18200 this diode is
intrinsic. Reverse current is actually shared between the
power switch and the diode due to the fact that the DMOS
switch can conduct current in either direction. For current
levels less than 2A to 2.5A the voltage across the power
switch, (IxR
DS(on)
), is less than the forward threshold voltage
of the diode and all of the current flows through the switch. At
higher current levels the diode conducts and the current is
shared.
An important consideration in the design of the LMD18200
was to make sure that the power switches could handle not
only the load current but also the additional reverse recovery
current of the protection diodes. This is illustrated in Figure 4
where switch A1 is initially ON and conducting reverse cur-
rent. At the interval when A1 is commanded OFF and the
lower switch in the same leg of the H-Bridge, A2 is com-
manded ON, a short deadtime (purposely built in to the
LMD18200 to eliminate shoot-through currents) occurs.
During this time current begins to flow through the protection
diode across switch A1. When switch A2 comes ON, the
diode becomes reverse biased. Switch A2 must then con-
duct the load current plus the reverse recovery current of the
diode for the short (approximately 100 ns) reverse recovery
time of the diode. This additional requirement on the power
switches has been accommodated in the design of the
LMD18200.
CURRENT SENSING
A unique feature of the LMD18200 is circuitry that allows for
the sensing of the current through the load without affecting
the supply or ground return lines. A common method for
sensing the load current is to insert a small valued power
resistor in series with either the V
CC
supply or ground lines
and detect the voltage drop across this resistor. This voltage
drop not only takes away from the available voltage to be
applied to the load but is also somewhat difficult to amplify
due to very low or possibly fast varying common mode
voltage presented to the amplifier.
The principle employed in the LMD18200 is the same as that
used in discrete current sensing power MOSFETs. Each
DMOS power transistor is actually comprised of many
smaller cells connected in parallel. Due to the positive tem-
perature coefficient of the ON resistance of each cell, the
total current through the switch divides almost equally be-
tween the individual cells. A few of these cells are separated
out to provide a current that is a scaled down replica of the
total switch current. Figure 5 shows a simplified functional
diagram of the current sensing circuitry.
The current sourced by the Current Sense Output pin is a
current proportional to the sum of the total forward current
conducted by the two upper DMOS switches of the H-Bridge.
This sense current has a typical value of 377 µA per Amp of
current through the power devices. Simply connecting a
resistor between the sense output pin and ground converts
this current to a voltage proportional to the current being
delivered to the load. This voltage is then suitable for feed-
back control or load over-current protection purposes.
CHARGE PUMP AND BOOTSTRAP CIRCUITRY
In order to drive a DMOS switch ON, its gate must be driven
approximately 10V more positive than its source voltage.
The lower switches of the H-Bridge have their source termi-
nals connected to ground and their gate drive is derived from
the V
S
supply voltage to the device. The two upper switches
however have their source terminals connected to the output
pins which are continually being switched between ground
and V
S
. In order to generate the gate drive voltage for these
switches a charge pump circuit is used. Figure 6a illustrates
this circuitry.
Transistors Q1 and Q2 are toggled at an internally generated
clock frequency of 300 kHz. When Q2 is ON, the on-chip
charge pump capacitor, C
CP
, is charged to approximately
14V. When Q1 is switched ON the bottom of this capacitor is
connected to the supply voltage, V
S
. This causes the voltage
at point X, which connects to the gate of the upper DMOS
power switch, to rise to about 14V more positive than the
supply. This ensures that the upper device switches ON
even if its source is at the V
S
potential.
01085905
FIGURE 5. The Current Sensing
Circuitry of the LMD18200
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3 Key Features
(Continued)
Capacitor C
CP
is limited in value for practical considerations.
Due to the limited charge that can be stored in C
CP
the
turn-on time of the upper DMOS transistors is relatively slow
but nevertheless satisfactory for operating frequencies up to
around 1 kHz. Once the DMOS device is turned ON the
300 kHz oscillator keeps the charge pump circuit running
thereby holding the power device ON as long as it is com-
manded by the input control to do so. This charge pump
circuit takes care of all the necessary voltage conditioning
required by the DMOS transistors so that the external logic
control applied to the LMD18200 can be simple TTL com-
patible signals.
For higher frequency operation, faster turn-on of the upper
DMOS switches is necessary. This can be obtained through
the use of external bootstrap capacitors. The bootstrap cir-
cuit is shown in Figure 6b. The operating principle is similar
to that of the charge pump circuitry except that the switching
of the bootstrap capacitor, C
B
, is assumed by the DMOS
power switches of the H-Bridge itself. With plenty of current
available to charge these external capacitors they can have
a relatively large value (10 nF is recommended) and still be
charged in typically less than one microsecond. Since C
B
is
much larger than the