SA56 Datasheet, PDF (7/7 Pages) Cirrus Logic – PULSE WIDTH MODULATION AMPLIFIER

OPERATING
CONSIDERATIONS
SA56
4 QUADRANT DIGITAL MODE
During four-quadrant operation a single digital PWM input
includes magnitude and direction information. The digital PWM
input signal is applied to the DIR pin, as shown in Figure 4,
and the PWM pin is tied HIGH to V_DD. Both pairs of output
MOSFETs will switch in a locked, complementary fashion.
With a 50% duty cycle the average voltage of outputs
A_OUT and B_OUT will be the same, which is half of V_S so that the
average differential voltage over each period applied to the
load will therefore be zero.
Four-quadrant operation allows for smooth transitions
through zero current for low-speed applications. However,
power dissipation is slightly higher than in two-quadrant opera-
tion since all four output MOSFETs must switch every cycle.
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BRAKING – DIGITAL MODE
Under digital control, the SA56 can rapidly decelerate the
motor by shunting the winding currents through the output
MOSFETs. Logic LOW on the PWM input both A and B out-
puts high. The motor winding current circulates through the on
resistance of the MOSFETs quickly slowing the motor.
The winding current can be monitored with the I_SEN pin
during the braking of the motor. However, the current during
braking circulates in the normal forward direction through one
output MOSFET and is in the reverse in the other MOSFET.
The current sense feature can measure only forward currents.
The logic input on the DIR pin dictates which output MOSFET
is used for sensing the forward current during braking.
PROTECTION CIRCUITS
The most severe condition for any power device is a direct,
hard-wired ("screwdriver") short from an output to ground.
While the short-circuit protection will latch the output MOSFETs
within 500 ns (typical), the die and package may be required to
dissipate up to 500 Watts of power until the protection circuits
are activated.
This energy can be destructive, particularly at higher operat-
ing voltages, so sound thermal design is critical if fault tolerance
is to be established in the design. The V_S and P_GND pins may
become very hot during this period of high current.
Thermal and short-circuit protection are included in the
SA56 to prevent damage in the event that faults occur as
described below:
Short-circuit protection – The short-circuit protection circuits
will sense a direct short from either output (A_OUT or B_OUT) to
P_GND or V_S – as well as across the load. If the high-current
protection circuit engages, it will place all four MOSFETs in
the tristate state (high-impedance output). The SC output, pin
6, will go HIGH though not latch, thereby denoting that this
protection feature has been triggered.
Over-current protection – When the current on the high side
goes above 10 amperes peak, the over-current circuit tristates
so that the four MOSFETs go into a latched fault condition.
Thermal protection – The thermal protection circuits will en-
gage if the temperature of any of the four MOSFETs reaches
approximately 160°C. If this occurs, the FAULT output pin will
go HIGH. If the thermal protection circuit engages, it will place
all four MOSFETs in the tristate state (high-impedance output).
The T_LIM output which is normally LOW will go HIGH, though
not latch, thereby denoting which of the protection features
has been triggered.
PROGRAMMABLE CURRENT LIMIT
The I_SEN pin sources a current proportional to the forward
output current of the active P channel output MOSFET. The
proportionality is approximately 200 microamperes per ampere
of output current. Note that the I_SEN output is blocked during
the switching transitions when current spikes are likely to be
significant.
To create a programmable current limit, connect a resistor
from I_SEN to SIG_GND. If the voltage across this resistor exceeds
an internally-generated 2.75V threshold, all four output MOS-
FETs will be turned off for the remainder of the switching cycle.
A 2.7k-Ohm resistor will set the current limit at approximately
5 amperes.
The I_SEN output can also be used for maintaining a current
control loop in torque motor applications.
CURRENT SENSE LINEARITY CALCULATION
The current sense linearity is specified in the table on page
2 and is calculated using the method described below:
a) Define a straight line (y = mx + b) joining the two end data
points where, m is the slope and b is the offset or zero
crossover. Calculate the slope m and offset c using the
extreme data points. Assume I_sense in the y axis and I_load
in the x axis.
b) Calculate linear I_SEN (or ideal I_sense value, IS_IDEAL) using
the straight line equation derived in step (a) for the I_load
data points.
c) Determine deviation from linear I_SEN (step (b) and actual
measured I_sense value (IS_ACTUAL) as shown below:
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SA56U REV A FEBRUARY 2007 © 2007 Apex Microtechnology Corp.