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7

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

CHANNEL B: I

CHANNEL C: I

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.

Wheneverthereareno“fault”conditionsandtheinputcontrols

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.

OPERATING

CONSIDERATIONS

SA305