AD74115 datasheet(63/103 Pages) AD

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Data Sheet

AD74115

THEORY OF OPERATION

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Driving Inductive Loads

It is recommended to use the digital slew rate control when driving

inductive loads greater than approximately 4 mH. Controlling the

output slew rate minimizes ringing when stepping the output current

by minimizing the current rate of change (di/dt). See the IOUT typical

performance of the settling time with an inductive load with and

without the slew rate enabled in the Figure 11.

RESET FUNCTION

After the AD74115 is reset, all registers are reset to the default

state, and the calibration memory is refreshed. The device is config-

ured in high impedance mode. A reset can be initiated in several

ways.

The hardware reset is initiated by pulsing the RESET pin low. The

RESET pulse width must comply with the specifications in Table 14.

A software reset is initiated by writing the 0x15FA code (Software

Reset Key 1) followed by the 0xAF51 code (Software Reset Key 2)

to the CMD_KEY register.

A reset can also be initiated via the thermal reset function, which is

described in the Thermal Alert and Thermal Reset section.

If the VDLDO1V8 or the VDVCC drop below the specified power supply

monitors threshold highlighted in Table 13 the internal power-on

reset function resets the AD74115. The device does not come out

of reset until the VDLDO1V8 and the VDVCC rise above these voltage

levels.

After a reset cycle completes, the RESET_OCCURRED bit is set

in the ALERT_STATUS register. If an SPI transfer is attempted

before the reset cycle is complete (see Table 13 for typical reset

time), the CAL_MEM_ERR bit in the SUPPLY_ALERT_STATUS

fully refreshed. After the reset time elapses, clear these bits in the

ALERT_STATUS register before continuing to use the device.

FAULTS AND ALERTS

The AD74115 is equipped with several fault monitors to detect an

error condition.

If an alert or fault condition occurs, the ALERT pin asserts. To

determine the source of the alert condition, read the ALERT_

STATUS register. This register contains a latched bit for each alert

condition.

After the error condition is removed, clear the activated flag by writ-

ing 1 to the location of the corresponding bits in the ALERT_STA-

TUS register (write 0xFFFF to the ALERT_STATUS register to clear

all alert bits). Alerts asserted in SUPPLY_ALERT_STATUS must be

cleared before the ALERT_STATUS register.

The LIVE_STATUS register is a live representation of the error

conditions. The bits in this register are not latched and clear

automatically when the error condition is no longer present.

The ALERT_MASK register prevents error conditions from activat-

ing the ALERT pin.

Channel Faults

The AD74115 is equipped with multiple open-circuit and short-cir-

cuit faults in the various functions as described in the Device

Functions section. Manage faults as these faults appear and reset

the channel, if necessary, to avoid overheating the device.

Power Supply Monitors

The AD74115 includes six power supply monitors to detect a

supply failure. If any of the supplies fall to less than the defined

threshold detailed in Table 13, the corresponding bit is set in the

ALERT_STATUS register.

Thermal Alert and Thermal Reset

If the AD74115 die temperature reaches the alert temperature

described in Table 13, a high temperature error bit (TEMP_ALERT)

is set in the ALERT_STATUS register to alert the user of the

increasing die temperature.

The device can also be configured to reset at higher die temper-

atures. To reset the device at higher temperatures, enable the

thermal reset function by setting the EN_THERM_RST bit in the

THERM_RST register. After this bit is set, the device goes through

a full reset after the die temperature reaches the reset temperature

described in Table 13.

Burnout Currents

Burnout currents are used to verify the integrity of an attached

sensor and to ensure that it has not gone open circuit before taking

a measurement from it. The AD74115 can be enabled to provide

a user programmable, current source that can be programmed

to a fixed value between 50 nA and 10 μA. Burnout currents

are available on the VIOUT (to monitor the I/OP screw terminal),

SENSE_EXT1, and SENSE_EXT2 pins and can be programmed to

source or sink current.

The burnout current sources are disabled on power up. Program

the burnout current using the bits in the I_BURNOUT_CONFIG

Table 13.

The current source can be enabled at all times or alternatively

enabled when needed for diagnostic purposes. When a burnout

current source is enabled, the selected current is switched onto the

selected pin, and it flows in the external load.

FET LEAKAGE COMPENSATION

A software configurable input and output solution can include a

precision analog input and output capability along with a high

current, digital output capability on a single screw terminal. In this

case, the external FET used in the digital output function may

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