ADT6501/ADT6502/ADT6503/ADT6504

Data Sheet

Rev. B | Page 8 of 16

THEORY OF OPERATION

CIRCUIT INFORMATION

The ADT6501/ADT6502/ADT6503/ADT6504 are 11-bit digital

on-board temperature sensor generates a voltage precisely

proportional to absolute temperature, which is compared to

an internal voltage reference and input to a precision digital

modulator. The 12-bit output from the modulator is input into a

digital comparator where it is compared with a factory-set trip

level. The output trip pin is activated if the temperature measured

is greater than, in the case of the ADT6501/ADT6502, or less

than, in the case of the ADT6503/ADT6504, the factory-set trip

level. Overall accuracy for the ADT650x family is ±6°C

(maximum) from −45°C to +115°C.

The on-board temperature sensor has excellent accuracy and

linearity over the entire rated temperature range without needing

correction or calibration by the user. The ADT6501/ADT6503

have active low, open-drain output structures that can sink

current. The ADT6502/ADT6504 have active high, push-pull

output structures that can sink and source current. On power-

up, the output becomes active when the first conversion is

completed, which typically takes 30 ms.

The sensor output is digitized by a first-order, ∑-∆ modulator,

also known as the charge balance type analog-to-digital

converter (ADC). This type of converter utilizes time domain

oversampling and a high accuracy comparator to deliver 11 bits

of effective accuracy in an extremely compact circuit.

CONVERTER DETAILS

The Σ-Δ modulator consists of an input sampler, a summing

network, an integrator, a comparator, and a 1-bit digital-to-

analog converter (DAC). Similar to the voltage-to-frequency

converter, this architecture creates a negative feedback loop and

minimizes the integrator output by changing the duty cycle of

the comparator output in response to input voltage changes.

The comparator samples the output of the integrator at a much

higher rate than the input sampling frequency; this is called

oversampling. Oversampling spreads the quantization noise

over a much wider band than that of the input signal,

improving overall noise performance and increasing accuracy.

FACTORY-PROGRAMMED THRESHOLD RANGE

The ADT6501/ADT6502/ADT6503/ADT6504 are available

with factory-set threshold levels ranging from −45°C to +115°C

in 10°C temperature steps. The ADT6501/ADT6503 outputs are

intended to interface to reset inputs of microprocessors. The

ADT6502/ADT6504 are intended for driving circuits of

applications such as fan control circuits. Table 4 lists the

available temperature threshold ranges.

Table 4. Factory-Set Temperature Threshold Ranges

Device

ADT6501

ADT6502

ADT6503

ADT6504

HYSTERESIS INPUT

The HYST pin is used to select a temperature hysteresis of 2°C or

10°C. The digital comparator ensures excellent accuracy for the

of 10°C is selected. If the HYST pin is connected to GND, a

hysteresis of 2°C is selected. The HYST pin should not be left

floating. Hysteresis prevents oscillation on the output pin when

the temperature is approaching the trip point and after the

output pin is activated. For example, if the temperature trip is

45°C and the hysteresis selected is 10°C, the temperature would

have to go as low as 35°C before the output deactivates.

TEMPERATURE CONVERSION

The conversion clock for the part is generated internally. No

external clock is required. The internal clock oscillator runs an

automatic conversion sequence. During this automatic conversion

sequence, a conversion is initiated every 600 ms. At this time, the

part powers up its analog circuitry and performs a temperature

conversion.

This temperature conversion typically takes 30 ms, after which

the analog circuitry of the part automatically shuts down. The

analog circuitry powers up again 570 ms later, when the 600 ms

timer times out and the next conversion begins. The result of

the most recent temperature conversion is compared with the

factory-set trip point value. If the temperature measured is

greater than the trip point value, the output is activated. The

output is deactivated once the temperature crosses back over

the trip point threshold plus whatever temperature hysteresis is

selected. Figure 15 to Figure 18 show the transfer function for

the output trip pin of each generic model.