ADT7516 datasheet(26/44 Pages) AD

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ADT7516/ADT7517/ADT7519

Rev. B | 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 that is gained up by a switched capacitor amplifier

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 consumption.

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

Control Configuration 1 register 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 0x19) disables the round robin mode

and in turn sets up the single-channel mode. In single-channel

mode, only one channel (for example, the internal temperature

sensor) is measured in each conversion cycle.

The time taken to monitor all channels is normally not of

interest, because the most recently measured value can be read

at any time. For applications where the round robin time is

important, typical times at 25°C are given in the Specifications

section.

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 are 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 temperature

value register. Because both positive and negative temperatures

can be measured, the temperature data is stored in twos comple-

ment format, as shown in Table 9. The thermal characteristics

of the measurement sensor can 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 temperature

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

individual 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

ΔVBE = kT/q × ln(N)

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

temperature monitoring on some microprocessors, but it can

equally well be a discrete transistor.

If a discrete transistor is used, the collector is not grounded, and

should be linked to the base. If a PNP transistor is used, the

base is connected to the D− input and the emitter to the D+

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