MAX6666AUT+T datasheet(5/7 Pages) MAXIM

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Detailed Description

The MAX6666/MAX6667 are high-accuracy, low-cost, low

current (200µA typ) temperature sensors ideal for inter-

facing with µCs or µPs. The MAX6666/MAX6667 convert

the ambient temperature into a ratiometric PWM output at

a nominal frequency of 35Hz (±20%) at +25°C.

The time periods, t1 (high) and t2 (low) (Figure 1), are

easily read by the µP’s timer/counter port. To calculate the

temperature, use the expression below:

Temperature (°C) = +235 - (400 x t1)/t2

The µC or µP measures the output of the MAX6666/

MAX6667 by counting t1 and t2 and computing the tem-

perature based on their ratio. The resolution of the count

is a function of the processor clock frequency and the

resolution of the counter. The MAX6666/MAX6667 have a

resolution of approximately 11 bits. Always use the same

clock for t1 and t2 counters so that the temperature is

strictly based on a ratio of the two times, thus eliminating

errors due to different clocks’ frequencies.

The MAX6666 (Figure 2a) has a push-pull output and

provides rail-to-rail output drive. The ability to source and

sink current allows the MAX6666 to drive capacitive loads

up to 10nF with less than 1°C error.

The MAX6667 (Figure 2b) has an open-drain output. The

output capacitance should be minimized in MAX6667

applications because the sourcing current is set by the

pullup resistor. If the output capacitance becomes too

large, lengthy rise and fall times distort the pulse width,

resulting in inaccurate measurements.

Applications Information

Accurate temperature monitoring requires a good thermal

contact between the MAX6666/MAX6667 and the object

being monitored. A precise temperature measurement

depends on the thermal resistance between the object

being monitored and the MAX6666 die. Heat flows in

and out of plastic packages primarily through the leads.

For the best thermal contact, connect all unused pins to

ground. If the sensor is intended to measure the tempera-

ture of a heat-generating component on the circuit board,

mount the device as close as possible to that component

and share the ground traces (if they are not too noisy) with

the component. This maximizes the heat transfer from the

component to the sensor.

Power-Supply Bypassing

The MAX6666/MAX6667 operate from a +3V to +5.5V

supply. If a noisy power-supply line is used, bypass VCC

to GND with a 0.1µF capacitor.

Power Supply from µP Port Pin

The low quiescent current of the MAX6666/MAX6667

enables it to be powered from a logic line, which meets

the requirements for supply voltage range. This provides

a simple shutdown function to totally eliminate quiescent

current by taking the logic line low. The logic line must be

able to withstand the 0.1µF power-supply bypass capac-

itance.

Galvanic Isolation

Use an optocoupler to isolate the MAX6666/MAX6667

whenever a high common-mode voltage is present.

Because some optocouplers have turn-off times that are

much longer than their turn-on times, choose an opto-

coupler with equal turn-on and turn-off times. Unequal

turn-on/turn-off times produce an error in the temperature

reading.

Thermal Considerations

Self-heating may cause the temperature measurement

accuracy of the MAX6666/MAX6667 to degrade in some

applications. The quiescent dissipation and the power dis-

sipated by the digital output may cause errors in obtaining

the accurate temperature measurement. The temperature

errors depend on the thermal conductivity of the package

(SOT23, 140°C/W), the mounting technique, and the

airflow. Static dissipation in the MAX6666/MAX6667 is

typically 4.5mW operating at 5V with no load. As a worst-

case example, consider the MAX6667 and its maximum

rated load of 5mA and assume a maximum output voltage

of 0.8V adds 4mW power dissipation. Use Figure 3 to

estimate the temperature error.

Figure 1. MAX6666/MAX6667 PWM Output

www.maximintegrated.com

Maxim Integrated │ 5

MAX6666/MAX6667

High-Accuracy PWM Output Temperature Sensors

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