Allegro MicroSystems, LLC
115 Northeast Cutoff
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
Dual DMOS Full-Bridge
Microstepping PWM Motor Driver
A3972
8
V
REG
. This internally generated supply voltage is used to run
the sink-side DMOS outputs. V
REG
is internally monitored and
in the case of a fault condition, the outputs of the device are
disabled. The V
REG
pin should be decoupled with a 0.22 μF
capacitor to ground.
Current Regulation. The reference voltage can be set by ana-
log input to the REF terminal, or via the internal 2 V precision
reference. The choice of reference voltage and sense resistor set
the maximum trip current.
I
TRIPMAX
= V
REF
/(Range x R
S
)
Microstepping current levels are set according to the following
equations:
I
TRIP
= V
DAC
/(Range x R
S
)
V
DAC
= [(1 + DAC) x V
REF
]/64
where DAC input code equals 1 to 63 and Range is 4 or 8 as
selected by Word 0, D18. Programming the DAC input code to
zero disables the bridge, and results in minimum load current.
PWM Timer Function. The PWM timer is programmable via
the serial port to provide fi xed off-time PWM signals to the con-
trol block. In mixed-decay mode, the fi rst portion of the off time
operates in fast decay, until the fast-decay time count is reached,
followed by slow decay for the rest of the fi xed off-time period.
If the fast-decay time is set longer than the off-time, the device
effectively operates in fast-decay mode.
Oscillator. The PWM timer is based on an oscillator input,
typically 4 MHz. The A3972SB can be confi gured to select ei-
ther a 4 MHz internal oscillator or, if more precision is required,
an external clock can be connected to the OSC terminal. If an
external clock is used, three internal divider choices are select-
able via the serial port to allow fl exibility in choosing f
OSC
,
based on available system clocks. If the internal oscillator op-
tion is used, the absolute accuracy is dependent on the process
variation of resistance and capacitance. A precision resistor can
be connected from the OSC terminal to V
DD
to further improve
the tolerance. The frequency will be:
f
OSC
= 204 x 10
9
/R
OSC
If the internal oscillator is used without the external resistor, the
OSC terminal should be connected to ground.
Sleep Mode. The input terminal SLEEP is dedicated to putting
the device into a minimum current draw mode. When pulled
low, the serial port will be reset to all zeros and all circuits will
be disabled.
Shutdown. In the event of a fault due to excessive junction
temperature, or low voltage on V
CP
or V
REG
, the outputs of the
device are disabled until the fault condition is removed. At
power up, or in the event of low V
DD
, the UVLO circuit disables
the drivers and resets the data in the serial port to zeros.
Synchronous Rectifi cation. When a PWM off-cycle is
triggered, either by a bridge disable command or internal fi xed
off-time cycle, the load current will recirculate according to
the decay mode selected by the control logic. The A3972SB
synchronous rectifi cation feature will turn on the appropriate
MOSFET(s) during the current decay and effectively short out
the body diodes with the low r
DS(on)
driver. This will lower
power dissipation signifi cantly and can eliminate the need for
external Schottky diodes for most applications.
Four distinct modes of operation can be confi gured with the two
serial port control bits:
1. Active Mode. Prevents reversal of load current by turning
off synchronous rectifi cation when a zero current level is
detected.
2. Passive Mode. Allows reversal of current but will turn
off the synchronous rectifi er circuit if the load current inver-
sion ramps up to the current limit.
3. Disabled. MOSFET switching will not occur during load
recirculation. This setting would only be used with four
external clamp diodes per bridge.
4. Low Side Only. The low-side MOSFETs will switch on
during the off time to short out the current path through
the MOSFET body diode. With this setting, the high-side
MOSFETs will not synchronously rectify so four external
diodes from output to supply are recommended. This mode
is intended for use with high-power applications where it
is desired to save the expense of two external diodes per
bridge. In this mode, the sink-side MOSFETs are chopped
during the PWM off time. In all other cases, the source-side
MOSFETs are chopped in response to a PWM off com-
mand.
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FUNCTIONAL DESCRIPTION (continued)
