APEX MICROTECHNOLOGY CORPORATION • 5980 NORTH SHANNON ROAD • TUCSON, ARIZONA 85741 • USA • APPLICATIONS HOTLINE: 1 (800) 546-2739

6

PR

EL

IM

IN

AR

Y

OPERATING

CONSIDERATIONS

SA56

MODES OF OPERATION

The following chart shows the four modes of operation.

Mode

CPWM

PWM

DIR

Aout

Bout

2

Quad

Digital

GND

Modu-

lation

In

High

High

PWM

2

Quad

Digital

GND

Modu-

lation

In

Low

PWM

High

4

Quad

Digital

GND

High

Modu-

lated

In

DIR

DIR

4

Quad

Analog

Add

Cap. to

set

Fre-

quency

Drive

with

analog

signal

Not

used

but

GND

Greater

than 50%

high for a

low

input

Greater

than 50%

high for a

high

input

ANALOG INPUT OPERATION

The SA56 can operate with analog or digital inputs. In the

analog mode, the capacitor from CPWM to GND (SIG) sets

the frequency of an internal triangular ramp signal. An analog

input at the PWM pin is compared to the ramp to generate

the duty cycle of the output. In Analog mode, the digital input

on the DIR pin is ignored, though this pin should never be

left floating.

OPERATING WITH DIGITAL INPUTS

Two and 4 quadrant operation are possible with the SA56

when driven with a digital PWM signal from a microcontroller or

DSP. When using a digital modulation signal, tie the CPWM pin

to GND to disable the internal oscillator and ramp generator.

When operating in the digital mode, pulse widths should be

no less than 100ns and the switching frequency should remain

less than 500KHz. This will allow enough time for the output

MOSFETs to reach their full on/off state before receiving a

command to reverse state.

2 QUADRANT DIGITAL MODE

For sign/magnitude (2 quadrant) operation, two digital input

signals are required. A digital PWM signal to the PWM pin can

control the output duty cycle at one output pin with the other

output pin held "HIGH". The digital input on the DIR pin will

control direction by selecting the outputs that switch accord-

ing to the PWM input. If DIR is a logic "HIGH", the A output

will be held "HIGH" and the B output will be switched as the

inverse of the PWM input signal. If DIR is logic "LOW", the B

output will be held "HIGH" and the A output will be switched.

Operating in 2 quadrant mode reduces switching noise and

power dissipation, but limits the control of the motor at very

low speed.

A braking function can be achieved by holding the PWM input

"LOW", which will turn both of the upper MOSFETs on, rapidly

reducing the circulating current of the motor winding.

4 QUADRANT DIGITAL MODE

During 4 quadrant operation a single digital PWM input in-

cludes magnitude and direction information. The digital PWM

input signal is applied to the DIR pin and the PWM/INPUT pin

is tied to "HIGH". Both pairs of output MOSFETs will switch

in a locked anti-phase fashion from 0-100% duty cycle. With

a 50% duty cycle the average voltage of each output will be

half of Vs, and the differential voltage applied to the load will

be zero. Four quadrant operation allows smooth transitions

through zero current for position servos and low speed ap-

plications. Power dissipation is slightly higher since all four

output MOSFETs switch every cycle.