Dual DMOS Full-Bridge
Microstepping PWM Motor Driver
A3972
FUNCTIONAL DESCRIPTION (continued)
VREG. This internally generated supply voltage is used to run
the sink-side DMOS outputs. VREG is internally monitored and
in the case of a fault condition, the outputs of the device are
disabled. The VREG pin should be decoupled with a 0.22 μF
capacitor to ground.
Shutdown. In the event of a fault due to excessive junction
temperature, or low voltage on VCP or VREG, the outputs of the
device are disabled until the fault condition is removed. At
power up, or in the event of low VDD, the UVLO circuit disables
the drivers and resets the data in the serial port to zeros.
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.
Synchronous Rectification. When a PWM off-cycle is
triggered, either by a bridge disable command or internal fixed
off-time cycle, the load current will recirculate according to
the decay mode selected by the control logic. The A3972SB
synchronous rectification feature will turn on the appropriate
MOSFET(s) during the current decay and effectively short out
the body diodes with the low rDS(on) driver. This will lower
power dissipation significantly and can eliminate the need for
external Schottky diodes for most applications.
ITRIPMAX = VREF/(Range x RS)
Microstepping current levels are set according to the following
equations:
ITRIP = VDAC/(Range x RS)
VDAC = [(1 + DAC) x VREF]/64
Four distinct modes of operation can be configured with the two
serial port control bits:
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.
1. Active Mode. Prevents reversal of load current by turning
off synchronous rectification when a zero current level is
detected.
PWM Timer Function. The PWM timer is programmable via
the serial port to provide fixed off-time PWM signals to the con-
trol block. In mixed-decay mode, the first 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 fixed off-time period.
If the fast-decay time is set longer than the off-time, the device
effectively operates in fast-decay mode.
2. Passive Mode. Allows reversal of current but will turn
off the synchronous rectifier 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.
Oscillator. The PWM timer is based on an oscillator input,
typically 4 MHz. The A3972SB can be configured 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-
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.
able via the serial port to allow flexibility in choosing fOSC
,
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 VDD to further improve
the tolerance. The frequency will be:
fOSC = 204 x 109/ROSC
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.
continued next page ...
Allegro MicroSystems, LLC
115 Northeast Cutoff
8
Worcester, Massachusetts 01615-0036 U.S.A.
1.508.853.5000; www.allegromicro.com
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
