ADT7516 datasheet(26/44 Pages) AD

26 / 44 page

ADT7516/ADT7517/ADT7519

Rev. A | Page 26 of 44

Table 8. VDD Data Format (VREF = 2.28 V)

Digital Output

VDD Value (V)

Binary

Hex

2.7

01 1000 1011

18B

01 1011 0111

1B7

3.5

10 0000 0000

200

10 0100 1001

249

4.5

10 1001 0010

292

10 1101 1100

2DC

5.5

11 0010 0101

325

11 0110 1110

36E

6.5

11 1011 0111

3B7

11 1111 1111

3FF

On-Chip Reference

The ADT7516/ADT7517/ADT7519 have an on-chip 1.2 V band

gap reference, which is gained up by a switched capacitor ampli-

fier to give an output of 2.28 V. The amplifier is powered up for

the duration of the device monitoring phase and is powered

down once monitoring is disabled. This saves on current con-

sumption. The internal reference is used as the reference for the

ADC. The ADC is used for measuring VDD, internal temperature

sensor, external temperature sensor, and AIN inputs. The

internal reference is always used when measuring VDD, and the

internal and external temperature sensors. The external

reference is the default power-up reference for the DACs.

Round Robin Measurement

On power-up, the ADT7516/ADT7517/ADT7519 go into round

robin mode but monitoring is disabled. Setting Bit C0 of the

Configuration Register 1 to 1 enables conversions. It sequences

through all the available channels, taking a measurement from

each in the following order: VDD, internal temperature sensor,

external temperature sensor/(AIN1 and AIN2), AIN3, and

AIN4. Pin 7 and Pin 8 can be configured to be either external

temperature sensor pins or standalone analog input pins. Once

conversion is completed on the AIN4 channel, the device loops

around for another measurement cycle. This method of taking a

measurement on all the channels in one cycle is called round

robin. Setting Bit C4 of Control Configuration 2 (Address 19h)

disables the round robin mode and in turn sets up the single-

channel mode. The single-channel mode is where only one

channel, e.g., internal temperature sensor, is measured in each

conversion cycle.

The time taken to monitor all channels will normally not be of

interest, since the most recently measured value can be read at

any time. For applications where the round robin time is impor-

tant, typical times at 25°C are given in the specifications.

Single-Channel Measurement

Setting C4 of the Control Configuration 2 register enables the

single-channel mode and allows the ADT7516/ADT7517/

ADT7519 to focus on one channel only. A channel is selected by

writing to Bits C0:C2 in the Control Configuration 2 register.

For example, to select the VDD channel for monitoring, write to

the Control Configuration 2 register and set C4 to 1 (if not

done so already), then write all 0s to Bits C0:C2. All subsequent

conversions will be done on the VDD channel only. To change the

channel selection to the internal temperature channel, write to

the Control Configuration 2 register and set C0 = 1. When

measuring in single-channel mode, conversions on the channel

selected occur directly after each other. Any communication to

the ADT7516/ADT7517/ADT7519 stops the conversions, but

they are restarted once the read or write operation is completed.

Temperature Measurement Method

Internal Temperature Measurement

The ADT7516/ADT7517/ADT7519 contain an on-chip band

gap temperature sensor whose output is digitized by the on-chip

ADC. The temperature data is stored in the Internal Temper-

ature Value register. Because both positive and negative temper-

atures can be measured, the temperature data is stored in twos

complement format, as shown in Table 9. The thermal charac-

teristics of the measurement sensor could change and, therefore,

an offset is added to the measured value to enable the transfer

function to match the thermal characteristics. This offset is

added before the temperature data is stored. The offset value

used is stored in the internal temperature offset register.

External Temperature Measurement

The ADT7516/ADT7517/ADT7519 can measure the temper-

ature of one external diode sensor or diode-connected

transistor.

The forward voltage of a diode or diode-connected transistor,

operated at a constant current, exhibits a negative temperature

coefficient of about –2 mV/°C. Unfortunately, because the

absolute value of VBE varies from device to device, and indivi-

dual calibration is required to null this out, the technique is

unsuitable for mass production.

The technique used in the ADT7516/ADT7517/ADT7519 is to

measure the change in VBE when the device is operated at two

different currents. This is given by

( )

/ ×

∆

where:

K is Boltzmann’s constant.

q is the charge on the carrier.

T is the absolute temperature in kelvins.

N is the ratio of the two currents.

Figure 45 shows the input signal conditioning used to measure

the output of an external temperature sensor. This figure shows

the external sensor as a substrate transistor, provided for temp-

erature monitoring on some microprocessors, but it could

equally well be a discrete transistor.

ADT7516 Datasheet(PDF) 26 Page - Analog Devices

ADT7516 Datasheet(HTML) 26 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44

Datasheet Download

Link URL