3959
DMOS FULL-BRIDGE
PWM MOTOR DRIVER
FUNCTIONAL DESCRIPTION
V
REG
.
This internally generated voltage is used to operate
the sink-side DMOS outputs. The V
REG
terminal should
be decoupled with a 0.22
µF
capacitor to ground. V
REG
is
internally monitored and in the case of a fault condition,
the outputs of the device are disabled.
Charge Pump.
The charge pump is used to generate a
gate-supply voltage greater than V
BB
to drive the source-
side DMOS gates. A 0.22
µF
ceramic capacitor should be
connected between CP1 and CP2 for pumping purposes.
A 0.22
µF
ceramic capacitor should be connected between
CP and V
BB
to act as a reservoir to operate the high-side
DMOS devices. The CP voltage is internally monitored
and, in the case of a fault condition, the source outputs of
the device are disabled.
PHASE Logic.
The PHASE input terminal determines if
the device is operating in the “forward” or “reverse” state.
PHASE
0
1
OUT
A
Low
High
OUT
B
High
Low
EXT MODE Logic.
When using external PWM current
control, the EXT MODE input determines the current path
during the chopped cycle. With EXT MODE low, fast
decay mode, the opposite pair of selected outputs will be
enabled during the off cycle. With EXT MODE high,
slow decay mode, both sink drivers are on with ENABLE
low.
EXT MODE
0
1
Decay
Fast
Slow
Current Regulation.
Load current is regulated by an
internal fixed off-time PWM control circuit. When the
outputs of the DMOS H bridge are turned on, the current
increases in the motor winding until it reaches a trip value
determined by the external sense resistor (R
S
) and the
applied analog reference voltage (V
REF
):
I
TRIP
= V
REF
/10R
S
At the trip point, the sense comparator resets the source-
enable latch, turning off the source driver. The load
inductance then causes the current to recirculate for the
fixed off-time period. The current path during
recirculation is determined by the configuration of slow/
mixed/fast current-decay mode via PFD1 and PFD2.
Oscillator.
The PWM timer is based on an internal
oscillator set by a resistor connected from the R
OSC
terminal to V
DD
. Typical value of 4 MHz is set with a
51 kΩ resistor. The allowable range of the resistor is from
20 kΩ to 100 kΩ.
f
OSC
= 204 x 10
9
/R
OSC
.
If R
OSC
is not pulled up to V
DD
, it must be shorted to
ground.
Fixed Off Time.
The A3959 is set for a fixed off time of
96 cycles of the internal oscillator, typically 24
µs
with a
4 MHz oscillator.
ENABLE Logic.
The ENABLE input terminal allows
external PWM. ENABLE high turns on the selected sink-
source pair. ENABLE low switches off the source driver
or the source and sink driver, depending on EXT MODE,
and the load current decays. If ENABLE is kept high, the
current will rise until it reaches the level set by the internal
current-control circuit.
ENABLE
0
1
Outputs
Chopped
On
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