Detailed Description

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

low current (200µA typ) temperature sensors ideal for

interfacing 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.

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

the temperature, use the expression below:

The µC or µP measures the output of the MAX6666/

temperature 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.

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 capaci-

tive 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 ther-

mal contact between the MAX6666/MAX6667 and the

object being monitored. A precise temperature mea-

surement 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 temperature of a heat-generating compo-

nent 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

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 pro-

vides 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 capacitance.

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

optocoupler 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 dissipated by the digital output may cause

errors in obtaining the accurate temperature measure-

ment. 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 temper-

ature error.

High-Accuracy PWM Output Temperature

Sensors

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Figure 1. MAX6666/MAX6667 PWM Output