
18
Linear Technology Magazine • February 1993
DESIGN IDEAS
The LT1158 half-bridge motor driver
incorporates a number of powerful
protection features. Some of these,
such as its adaptive gate drive, are
dedicated in function. Others are open
to a variety of uses, depending upon
application requirements. The circuit
shown in Figure 1 takes advantage of
the wide common-mode input range of
the LT1158’s FAULT comparator to
perform ground-referenced current
sensing in an H-bridge motor driver.
By using ground-referenced sensing,
protection can easily be provided
against overloaded, stalled, or shorted
motors. For overloads and stalls, the
circuit becomes a constant-current
chopper, regulating the motor’s arma-
ture current to a preset maximum
value. For shorted loads, the circuit
protects itself by operating at a very
low duty cycle until the short is cleared.
Setting Up For Ground-
Referenced Sensing
The circuit of Figure 1 is essentially
a straightforward LT1158 H-bridge, of
the “sign/magnitude” type. (See the
LT1158 data sheet for a description of
component functions.) In many
LT1158 applications, a current-sense
resistor is placed in each upper
MOSFET source lead. This circuit, how-
ever, senses the IR drop across one
resistor (R1) common to the sources of
both lower MOSFETs. In Figure 1,
U1’s FAULT output activates the con-
stant-current protection mode (for
overloads and stalls), and U2’s FAULT
output indicates a shorted load. Hence,
given a maximum continuous motor
current of 15A, R1’s value is easily
determined: V
SENSE(+)
of U1 must ex-
ceed V
SENSE(–)
by the LT1158’s internal
threshold of 110mV in order for FAULT
to go low. 15A x R1 = 0.110V, so R1 =
(0.110V/15A) ≅7.5mΩ. The FAULT pin
of U2 should go low when I
R1
is 24A, so
a 1.6:1 voltage divider is added at U2’s
SENSE(+) input. R2, R3, C1, and C2
filter any switching spikes which ap-
pear across R1.
Closing the Loop on Overloads
If the motor is overloaded or stalled,
its back EMF will drop, causing the
armature current to increase at a rate
determined primarily by the motor’s
inductance. Without protection, this
current could rise to a value limited
only by supply voltage and circuit re-
sistance. The necessary protection is
provided via the feedback loop formed
by U1’s FAULT output, U3A, U4B, and
U4D. When I
R1
exceeds 15A, the FAULT
pin of U1 conducts, triggering the 40µs
monostable U3A. The “Q” output of
U3A in turn forces the outputs of U4B
and U4D to a logic low state, turning off
Q1 or Q3, and turning on both Q2 and
Q4. For the time during which U3A’s
“Q” output is high, the motor current
decays through the path formed by the
motor’s resistance, plus the “on” resis-
tance of Q2 and Q4 in series. In this
application, turning both lower MOS-
FETs on is preferable to forcing all four
MOSFETs off, as it provides a low-
resistance recirculation path for the
motor current. This reduces motor and
supply ripple currents, as well as MOS-
FET dissipation. At the end of U3A’s
40µs timeout, the H-bridge turns on
again. If the overload still exists, the
current quickly builds up to the U1
“FAULT” trip point again, and the 40ms
timeout repeats. This feedback loop
holds the motor current approximately
constant at 15A for any combination of
supply voltage and duty cycle that
would otherwise cause an excess cur-
rent condition. When the motors cur-
rent draw falls below 15A, the circuit
resumes normal operation.
LT1158 H-Bridge uses Ground-
Referenced Current Sensing
for System Protection by Peter Schwartz
Opening the Loop on Shorts
In the event of a short across the
motor terminals, the current through
the H-bridge rises faster than the U1/
U3A loop can regulate it. This could
easily exceed the safe operating area
limits of the MOSFETs. The solution is
simple: when the FAULT comparator
of U2 detects that I
R1
≥ 24A, monostable
U3B is triggered. The “Q’” output of
U3B will then hold the ENABLE line of
the two LT1158s “low” for 10ms, re-
sulting in a rapid shutdown and a very
low duty cycle. After the 10ms shut-
down interval, U3B’s “Q’” output will
return high, and the bridge will be re-
enabled. If the motor remains shorted,
U3B is triggered again, causing an-
other 10ms shutdown. When the short
is cleared, circuit operation returns to
that described above.
A Final Note
As a class, sign/magnitude H-bridge
systems are susceptible to MOSFET
and/or motor damage if the motor
velocity is accelerated rapidly, or the
state of the DIRECTION line is
switched while the motor is rotating.
This is especially true if the motor/
load system has high inertia. The cir-
cuit of Figure 1 is designed to provide
protection under these conditions: the
motor may be commanded to acceler-
ate and to change direction with no
precautions. For the case of decelera-
tion, however, it’s generally best to use
a controlled velocity profile. If a spe-
cific application requires the ability to
operate with no restrictions upon the
rate of change of duty cycle, there are
straightforward modifications to Fig-
ure 1 which allow this. Please contact
the factory for more information.