A4982
DMOS Microstepping Driver with Translator
and Overcurrent Protection
overcurrent condition that exists. This causes the driver to go into
a fixed off-time cycle. After the fixed off-time expires the driver
turns on again and the process repeats. In this condition the driver
is completely protected against overcurrent events, but the short
is repetitive with a period equal to the fixed off-time of the driver.
This condition is shown in figure 5.
If the driver is operating in Mixed decay mode, it is normal for
the positive current to spike, due to the bridge going in the for-
ward direction and then in the negative direction, as a result of the
direction change implemented by the Mixed decay feature. This
is shown in figure 6. In both instances the overcurrent circuitry is
protecting the driver and prevents damage to the device.
The DAC output reduces the V
REF
output to the current sense
comparator in precise steps, such that
I
trip
= (%I
TripMAX
/ 100)
×
I
TripMAX
(See table 2 for %I
TripMAX
at each step.)
It is critical that the maximum rating (0.5 V) on the SENSE1 and
SENSE2 pins is not exceeded.
Fixed Off-Time.
The internal PWM current control circuitry
uses a one-shot circuit to control the duration of time that the
DMOS FETs remain off. The off-time, t
OFF
, is determined by the
ROSC terminal. The ROSC terminal has three settings:
▪
ROSC tied to VDD — off-time internally set to 30
μs,
decay
mode is automatic Mixed decay except when in full step where
decay mode is set to Slow decay
▪
ROSC tied directly to ground — off-time internally set to
30
μs,
current decay is set to Mixed decay for both increasing
and decreasing currents, except in full step where decay mode
is set to Slow decay. (See Low Current Microstepping section.)
▪
ROSC through a resistor to ground — off-time is determined
by the following formula , the decay mode is automatic Mixed
decay for all step modes.
t
OFF
≈
R
OSC
⁄
825
Where t
OFF
is in
μs.
Charge Pump
(CP1 and CP2).
The charge pump is used to
generate a gate supply greater than that of VBB for driving the
source-side FET gates. A 0.1
μF
ceramic capacitor, should be
connected between CP1 and CP2. In addition, a 0.1
μF
ceramic
capacitor is required between VCP and VBB, to act as a reservoir
for operating the high-side FET gates.
Capacitor values should be Class 2 dielectric ±15% maximum,
or tolerance R, according to EIA (Electronic Industries Alliance)
specifications.
ate the sink-side FET outputs. The VREG pin must be decoupled
with a 0.22
μF
ceramic capacitor to ground. V
REG
is internally
monitored. In the case of a fault condition, the FET outputs of the
A4982 are disabled.
Capacitor values should be Class 2 dielectric ±15% maximum,
or tolerance R, according to EIA (Electronic Industries Alliance)
specifications.
¯ ¯ ¯ ¯¯ ¯
Enable Input
(
¯ ¯ ¯ ¯ ¯¯
)
.
This input turns on or off all of the
ENABLE
FET outputs. When set to a logic high, the outputs are disabled.
When set to a logic low, the internal control enables the outputs as
required. The translator inputs STEP, DIR, MS1, and MS2, as well
as the internal sequencing logic, all remain active, independent of
¯ ¯ ¯ ¯¯¯
the ¯ ¯ ¯ ¯ ¯ ¯ input state.
ENABLE
V
REG
(VREG)
.
This internally-generated voltage is used to oper-
Blanking.
This function blanks the output of the current sense
comparators when the outputs are switched by the internal current
control circuitry. The comparator outputs are blanked to prevent
false overcurrent detection due to reverse recovery currents of the
clamp diodes, and switching transients related to the capacitance
of the load. The blank time, t
BLANK
(μs), is approximately
t
BLANK
≈
1
μs
Shorted-Load and Short-to-Ground Protection.
If the motor leads are shorted together, or if one of the leads is
shorted to ground, the driver will protect itself by sensing the
overcurrent event and disabling the driver that is shorted, protect-
ing the device from damage. In the case of a short-to-ground, the
¯LEEP
¯¯¯
device will remain disabled (latched) until the S¯ ¯ ¯ ¯ input goes
high or VDD power is removed. A short-to-ground overcurrent
event is shown in figure 4.
or an undervoltage (on VCP), the FET outputs of the A4982 are
disabled until the fault condition is removed. At power-on, the
When the two outputs are shorted together, the current path is
through the sense resistor. After the blanking time (≈1
μs)
expires, UVLO (undervoltage lockout) circuit disables the FET outputs
the sense resistor voltage is exceeding its trip value, due to the
and resets the translator to the Home state.
Shutdown.
In the event of a fault, overtemperature (excess T
J
)
Allegro MicroSystems, Inc.
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
9
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
