© 2001 IXYS/DEI All rights reserved

4

IXDP 610

Description

Introduction

The IXDP610 is a digital PWM con-

troller. It simplifies the interface

between a microprocessor and a

switching power bridge by providing to

a micro-processor the means to directly

control the average voltage across a

load (DC motor, etc.). Since the

IXDP610 generates two

complementary PWM control signals,

there is no need for Digital to Analog

Converters (DACs), Sawtooth

Generators, and Analog Comparators.

OUT1 and OUT2 can directly drive the

buffers to the power transistors.

Use of the IXDP610 in a DC servo

system is depicted in the system block

diagram shown in Fig. 1. The IXDP610

receives digital data from the micropro-

cessor and converts the data to a pair

of complementary PWM signals that

can be used to control the average

voltage across a DC servo motor. A

Shaft Encoder Peripheral Interface

(SEPI) IC converts incremental

encoder signals to a binary number so

the micro-processor can monitor and

motor.

It is possible to generate PWM control

signals in software with a dedicated

microprocessor or microcontroller. This

has the limitation, however, of very low

switching frequencies (<5 kHz) and

significant software overhead. By using

the IXDP610 to handle the generation

of the PWM control signals, a micro-

processor can handle several PWM

channels and the PWM control signals

can switch at relatively high rates (up to

300 kHz). Servicing the IXDP610 is a

simple as writing an 8-bit number to the

Pulse Width latch whenever a change

in duty cycle is desired. This is analo-

gous to writing data to a DAC.

Programmable dead-time

Because the IXDP610 is a digital IC,

and is programmable, it is possible to

tailor the dead-time period (defined as

t

dead-time feature is difficult to

duplicate in the equivalent analog

system. The control of a switching

bridge usually involves a process of

alternating the “on-time” of two power

switches connected in series between

a high-voltage and a low-voltage. For

example, the H-bridge of Fig. 3 can be

operated by turning the upper left and

lower right transistors on and leaving

the two remaining transistors off, during

the first half of the PWM cycle. In the

second half of the cycle, the upper right

and lower left transistors are on and

Fig. 1 Basic System Configuration

Fig. 3 IXDP610 to DC Servo Motor Full Bridge Block

Diagram

guaranteeing that both transistors in a

leg are off for a minimum of time during

a transition (the dead-time period).

Since the dead-time is programmable,

it can be tailored to the specific

application. It can be short for high-

speed MOSFETs and longer for IGBTs.

Protection circuitry

The IXDP610 has several features that

facilitate protection of the power devi-

ces being controlled. The ODIS pin is

an input that can be driven by external

hardware under emergency shutdown

conditions, such as over-current and

over-temperature. The Stop bit, in the

Control latch, provides a mechanism

through which the software can indefi-

nitely disable the complementary out-

puts. ODIS and Stop perform similar

functions, they provide a means to

protect the power devices from measu-

rable system hazards such as over-

current, over-voltage, over-temperature

etc.

Software runaway is a system hazard

that is difficult or impossible to

measure. The Lock bit, in the Control

latch, can be used to protect the

system from software runaway and/or

errors. Setting the Lock bit prevents

subsequent writes to the Control latch

from having any affect on the

IXDP610’s operating para-meters.

Setting the Lock bit does not prevent

one from asserting the Stop bit. Once

the Lock bit is set, it is impossible to

modify critical parameters, such as the

dead-time delay or the PWM wave-

form resolution.

Control latch

The Control latch is composed of eight

bits that determine the IXDP610’s

Fig. 2 PWM Cycle Time and Dead-

Time Definition

the remaining two are off. During the

transition, between the first half and the

second half of the PWM cycle, there is

a very short period of time when both

the upper transistor and the lower

transistor in a leg could be on. If both

transistors are on, for this short period

of time, they will effectively short the

high voltage supply to ground-this is an

undesirable situation.

The IXDP610’s programmable dead-

time feature prevents this situation by

SEPI