2-Phase Stepper Motor Bipolar Driver ICs (2-Phase/1-2 Phase Excitation)
A3952SB/SLB/SW
sAPPLICATION
NOTES
(A) Current Sensing
The actual peak load current (I
OUTP
) will be greater than the cal-
culated value of I
TRIP
due to delays in the turn OFF of the driv-
ers. The amount of overshoot can be approximated as
I
OUTP
(V
BB
−
[(I
TRIP
•
R
LOAD
)+V
BEMF
])
•
t
pd (pwm)
L
LOAD
The thermal performance in applications with high load currents
and/or high duty cycles can be improved by adding external
diodes in parallel with the internal diodes. In internal PWM slow-
decay applications, only the tow top-side (flyback) diodes need
be added. For internal fast-decay PWM, or external PHASE or
ENABLE input PWM applications, all four external diodes should
be added for maximum junction temperature reduction.
(C)PCB Layout
The load supply terminal, V
BB
, should be decoupled (>47
µ
F elec-
trolytic and 0.1
µ
F ceramic capacitors are recommended) as
close to the device as is physically practical. To minimize the
effect of system ground I
•
R drops on the logic and reference
input signals, the system ground should have a low-resistance
return to the load supply voltage.
See also “Current Sensing” and “Thermal Considerations” above.
(D)Fixed Off-Time Selection
With increasing values of t
off
, switching losses decrease, low-
level load-current regulation improves, EMI is reduced, the PWM
frequency will decrease, and ripple current will increase. The
value of t
off
can be chosen for optimization of these parameters.
For applications where audible noise is a concern, typical val-
ues of t
off
are chosen to be in the range of 15 to 35
µ
s.
(E) Stepper Motor Applications
The MODE terminal can be used to optimize the performance
of the device in microstepping/sinusoidal stepper motor drive
applications. When the average load current is increasing, slow-
decay mode is used to limit the switching losses in the device
and iron losses in the motor.
This also improves the maximum rate at which the load current
can increase (as compared to fast decay) due to the slow rate
of decay during t
off
. When the average load current is decreas-
ing, fast-decay mode is used to regulate the load current to the
desired level. This prevents tailing of the current profile caused
by the back-EMF voltage of the stepper motor.
In stepper motor applications applying a constant current to the
load, slow-decay mode PWM is used typically to limit the switch-
ing losses in the device and iron losses in the motor.
where V
BB
is the load/motor supply voltage, V
BEMF
is the back-
EMF voltage of the load, R
LOAD
and L
LOAD
are the resistance and
inductance of the load respectively, and t
pd (pwm)
is the propaga-
tion delay as specified in the electrical characteristics table.
The reference terminal has an equivalent input resistance of
50kΩ±30%. This should be taken into account when determin-
ing the impedance of the external circuit that sets the reference
voltage value.
To minimize current-sensing inaccuracies caused by ground
trace IR drops, the current-sensing resistor should have a sepa-
rate return to the ground terminal of the device. For low-value
sense resistors, the IR drops in the PCB can be significant and
should be taken into account. The use of sockets should be
avoided as their contact resistance can cause variations in the
effective value of R
S
.
Larger values of R
S
reduce the aforementioned effects but can
result in excessive heating and power loss in the sense resistor.
The selected value of R
S
must not cause the SENSE terminal
absolute maximum voltage rating to be exceeded. The recom-
mended value of R
S
is in the range of
R
S
(0.375 to 1.125)
I
TRIP
The current-sensing comparator functions down to ground al-
lowing the device to be used in microstepping, sinusoidal, and
other varying current profile applications.
(B) Thermal Considerations
For reliable operation, it is recommended that the maximum
junction temperature be kept as low as possible, typically 90°C
to 125°C. The junction temperature can be measured by at-
taching a thermocouple to the power tab/batwing of the device
and measuring the tab temperature, T
T
. The junction tempera-
ture can then be approximated by using the formula
T
J
T
T
+ (2V
F
I
OUT
R
θ
JT
)
where V
F
is the clamp diode forward voltage and can be deter-
mined from the electrical specification table for the given level
of I
OUT
. The value for R
θ
JT
is given in the package thermal resis-
tance table for the appropriate package.
The power dissipation of the batwing packages can be improved
by 20 to 30% by adding a section of printed circuit board copper
(typically 6 to 18 square centimeters) connected to the batwing
terminals of the device.
A3952SB/SLB/SW
75