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

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OPERATING

CONSIDERATIONS

SA56

PROTECTION CIRCUITS

CURRENT SENSE LINEARITY CALCULATION

Thermal and short circuit protection are included in the

SA56 to prevent damage during fault conditions. High current

protection circuits will sense a direct short from either output

to GND or Vs as well as across the load. The thermal protec-

tion will engage when the temperature of the MOSFETs reach

approximately 160Â°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 SC or TLIM

output will also go "HIGH", to indicate which of the protection

features has been triggered. The fault going high disables the

The current sense linearity is calculated using the method

described below:

a) Define straight line (y = mx + c) joining the two end data

points where, m is the slope and c is the offset or zero

crossover. Calculate the slope m and offset c using the

extreme data points. Assume Isense in the y axis and Iload

in the x axis.

b) Calculate linear Isense (or ideal Isense value, ISIDEAL) using

the straight line equation derived in step (a) for the Iload

data points.

4 output transistors. To reset the fault condition, cycle the VDD

c) Determine deviation from linear Isense (step (b)) and actual

power or bring the DISABLE pin "HIGH" then "LOW".

measured Isense value (ISACTUAL) as shown below:

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

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within 250ns (typical) the die and package may be required

to dissipate up to 600 Watts 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.

Y PROGRAMMABLE CURRENT LIMIT

The ISEN pin sources a current proportional to the forward

output current of the active P channel output MOSFET. The

proportionality is 300ÂµA (nom) per ampere of output current.

R The ISEN output is blocked during the switching transitions

when current spikes can be significant.

To create a programmable current limit, connect a resis-

A tor from ISEN out to GND. When the voltage across this

resistor exceeds internally generated 2.75V threshold, all 4

output MOSFETs will be turned off for the remainder of the

switching cycle. A 2.75KÎ© resistor will set the current limit to

I N approximately 5 Amps.

The ISEN output can also be used for maintaining a current

P R E L I M control loop in torque motor applications.

IC rev C errata information

This document describes the errata information for SA56 rev

C full H-Bridge DC motor driver. Rev C parts can be identified

by date code 0206 marked on the EX package.

Errata Number Description

and Date

Impact

Dated: 3/3/06

TLIM pin:

This pin is mod-

ified to serve

as a flag for

any fault occur-

rence including

short-circuit,

over current

and over tem-

perature.

Impact:

Grounding the TLIM pin

disables all fault protec-

tion mechanisms in the

SA56 including SC, over

current and over temp.

This pin should be left

floating at all times un-

less the user desires

to disable all protection

mechanisms.

Note: The errata items described in the table above are

strictly for beta samples and will be rectified to conform to

SA56U specifications for the production parts.

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