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
temperature sensors with a 12
th
bit acting as the sign bit. An
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 Threshold (T
) Range
ADT6501 +35°C < T
< +115°C
ADT6502 +35°C < T
< +115°C
ADT6503 −45°C < T
< +15°C
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
hysteresis value. If the HYST pin is connected to V
CC
, a hysteresis
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.
