SA305_07 Datasheet, PDF (7/8 Pages) Cirrus Logic – PULSE WIDTH MODULATION AMPLIFIER

OPERATING
CONSIDERATIONS
SA305
GENERAL
Please read Apex Application Note 1 “General Operat-
ing Considerations” which covers stability, power supplies,
heat sinking, mounting, current limit, SOA interpretation, and
specification interpretation. Visit www.apexmicrotech.com for
design tools that help automate tasks such as calculations
for stability, internal power dissipation, current limit, heat sink
selection, Apex’s complete Application Notes library, Technical
Seminar Workbook and Evaluation Kits.
GROUND PINS
Analog and Power Grounds should be connected externally
at only one point on the motor control board in such a way
that there is no current flow through the connection to avoid
noise related issues.
PROTECTION
Each of the six output devices includes short circuit protec-
tion to prevent damage from direct shorts to GND or VS. The
SA305 is protected against overheating with built in thermal
monitoring. The thermal protection will engage when the tem-
perature of the MOSFETs reach approximately 160°C. The
FAULT output pin will go “HIGH” if either protection circuits
engages and will place all MOSFETs in the “OFF” state (high
impedance output). 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 off the die and package may be required
to dissipate a large amount of power until the protection is
engaged. This energy can be destructive, particularly at higher
operating voltages, so good thermal design is critical if such
fault tolerance is required of the system.
The SA305 has an internal FAULT latch mechanism by
which the device stays disabled (in case a fault occurs) un-
less the user resets it. If the SA305 goes into FAULT condition
because of short-circuit, over current or high temperature, the
DISABLE pin needs to be pulled HIGH (a brief 200ns or more
pulse should suffice) to reset the SA305 and resume normal
operation. However, before resetting the SA305 the user has to
ensure that the FAULT has been eliminated. Please note that
under voltage lockout does not set the internal fault latch.
CONTROL
Each output MOSFET is controlled by a single input. There
is a provision inside the SA305 to prevent the upper and lower
FET of the same channel from being active at the same time
even though the input controls request that both the N and P
devices from one half bridge be on.
POWER SUPPLY BYPASSING
Bypass capacitors to power supply terminals +Vs and –Vs
must be connected physically close to the pins to prevent lo-
cal parasitic oscillation in the output stage of the SA305. Use
electrolytic capacitors at least 10μF per output amp required.
Bypass the electrolytic capacitors with high quality ceramic
capacitors (X7R) 0.1μF or greater. See the external connec-
tions diagram on page 1.
CURRENT SENSE
The current of each phase can be read using the IMON output
pins. The high side of each half bridge current is monitored
separately. The current sense output level is as follows:
CHANNEL A: I_{SENSE_A} = I_O /4148 + 25µA
CHANNEL B: I_{SENSE_B} = I_O /3491 + 30µA
CHANNEL C: I_{SENSE_C} = I_O /3819 + 35µA
External power current sense resistors are not required with
the SA305. However, in order to read the current level using a
standard A/D input a resistor of 1KΩ should be shunted across
each output. A standard 1/4W resistor is sufficient here. Motor
current adjustments are made through the PWM inputs. Above
the internal limit the device self-protects.
EXTERNAL SCHOTTKY DIODES
External schottky diodes are required because of superior
reverse recovery characteristics compared to the internal
body diodes.
SA305 OPERATION
The SA305 is used to drive three phase motors but can
be used where ever three high current outputs are required.
A DSP or microcontroller is used to control and monitor the
operation of the SA305.
The current through each of the three P channel drive
transistors is monitored by on-board circuitry. Current is set
using the PWM inputs which drive each FET independently.
Once the desired level is reached the inductance of the mo-
tor keeps the current near the programmed level. Should the
current get to the internally set 12A level, the driver is shutoff
to protect itself.
Whenever there are no “fault” conditions and the input controls
indicate an output should be on, the P and N drivers will turn
on. If the input controls are requiring that P-channel turn on
before the N-channel turns off, the SA305 will automatically
delay the P-channel turn on. The time between the N turning
off and the P turning on or the P channel turning off and the
N channel turning on is called dead time. An internally set
minimum dead time assures no “shoot through” current and
gives the clamp diode time to discharge.
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