Datasheet

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General Description

The MAX6682 converts an external thermistor’s temper-

ature-dependent resistance directly into digital form.

The thermistor and an external fixed resistor form a volt-

age-divider that is driven by the MAX6682’s internal

voltage reference. The MAX6682 measures the voltage

across the external resistor and produces a 10-bit +

sign output code dependent on that voltage.

The MAX6682 does not linearize the highly nonlinear

transfer function of a typical negative temperature coef-

ficient (NTC) thermistor, but it does provide linear out-

put data over limited temperature ranges when used

with an external resistor of the correct value. Over the

0° to +50°C temperature range, the MAX6682 produces

output data that is scaled to 8LSBs/°C (for 0.125°C res-

olution), provided that the correct thermistor and exter-

nal resistor values are used. Other temperature ranges

can be easily accommodated, but do not necessarily

yield data scaled to an even number of LSBs per

degree.

The 3-wire SPI™-compatible interface can be readily

connected to a variety of microcontrollers.

The MAX6682 is a read-only device, simplifying use in

systems where only temperature data is required.

Power-management circuitry reduces the average ther-

mistor current, minimizing self-heating. Between con-

versions, supply current is reduced to 21µA (typ). The

internal voltage reference is shut down between mea-

surements.

The MAX6682 is available in a small, 8-pin µMAX pack-

age and is specified over the -55°C to +125°C temper-

ature range.

Applications

HVAC

Medical Devices

Battery Packs/Chargers

Home Appliances

Features

♦ Converts Thermistor Temperature to Digital Data

♦ Low Average Thermistor Current Minimizes Self-

Heating Errors

♦ Low Supply Current, 21µA (typ) Including 10kΩ

Thermistor Current

♦ Internal Voltage Reference Isolates Thermistor

from Power-Supply Noise

♦ 10-Bit Resolution

♦ Accommodates Any Thermistor Temperature

Range

♦ Output Data Scaled for Direct Temperature

Readings from 0°C to +50°C

♦ Simple SPI-Compatible Interface

♦ Small, 8-Pin µMAX Package

MAX6682

Thermistor-to-Digital Converter

________________________________________________________________ Maxim Integrated Products 1

Ordering Information

19-2219; Rev 0; 2/02

For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at

1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com.

PART TEMP RANGE PIN-PACKAGE

MAX6682MUA -55°C to +125°C 8 µMAX

SPI is a trademark of Motorola, Inc.

Pin Configuration appears at end of data sheet.

3.3V

0.1µF

MC68HCXX

I/O

SCLK

MISO

SCLK

R+

R-

GND

MAX6682

EXT

THERMISTOR

Typical Operating Circuit

Summary of content (9 pages)

PAGE 1
9-2219; Rev 0; 2/02 Thermistor-to-Digital Converter Power-management circuitry reduces the average thermistor current, minimizing self-heating. Between conversions, supply current is reduced to 21µA (typ). The internal voltage reference is shut down between measurements. The MAX6682 is available in a small, 8-pin µMAX package and is specified over the -55°C to +125°C temperature range.
PAGE 2
MAX6682 Thermistor-to-Digital Converter ABSOLUTE MAXIMUM RATINGS Supply Voltage (VCC to GND) .................................-0.3V to +6V SO, SCK, CS, R-, R+ to GND ....................-0.3V to (VCC + 0.3V) R+ Current ........................................................................±20mA R- Current ...........................................................................±1mA SCK, CS, SO Current .........................................-1mA to +50mA ESD Protection (Human Body Model) ..............
PAGE 3
Thermistor-to-Digital Converter (VCC = 3V to 5.5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are specified at VCC = 3.3V and TA = +25°C.
PAGE 4
Thermistor-to-Digital Converter MAX6682 Pin Description PIN NAME 1 I.C. Internally Connected. Connect to GND or leave unconnected. FUNCTION 2 R+ Reference Voltage Output. External resistor positive input. 3 R- External Resistor Negative Input. Connect R- to the junction of the external resistor and the thermistor. 4 GND 5 CS Chip Select. Drive CS low to enable the serial interface. 6 SO Serial Data Output 7 SCK Serial Clock Input 8 VCC Positive Supply. Bypass VCC to GND with a 0.
PAGE 5
Thermistor-to-Digital Converter MAX6682 Table 1. Temperature vs. Digital Output for Standard Thermistor with REXT = 7680Ω THERMISTOR TEMPERATURE (°C) VREXT (mV) WITH STANDARD THERMISTOR AND REXT = 7680Ω* DECIMAL VALUE OF DOUT (1LSB = 0.125°C) DOUT +60.000 921.6 +55.875 001 1011 1111 +50.000 830.6 +48.625 001 1000 0101 +40.000 720.5 +40.000 001 0100 0000 +30.000 595.4 +30.125 000 1111 0001 +25.000 530.1 +25.000 000 1100 1000 +20.000 464.4 +19.875 000 1001 1111 +10.000 339.
PAGE 6
MAX6682 Thermistor-to-Digital Converter in the middle of the temperature range (+35°C for the 0°C to +70°C range). THERMISTOR NONLINEARITY vs. TEMPERATURE 3) Find REXT using the equation below: 0.25 0.20 REXT = LINEARITY ERROR (°C) 0.15 0.10 0.05 -0.05 -0.10 -0.15 -0.20 -0.25 5 10 15 20 25 30 35 40 45 TEMPERATURE (°C) Serial Interface Figure 3. Thermistor Nonlinearity vs.
PAGE 7
Thermistor-to-Digital Converter MAX6682 sider a 10kΩ (at +25°C) NTC thermistor in series with a 5110Ω resistor operating at +40°C with a constant 5V bias. If it is one of the standard thermistors in Table 2, its resistance is 5325Ω at this temperature. The power dissipated in the thermistor is: (5)2(5325) / (5325 + 5110)2 = 1.22mW This thermistor would therefore have a self-heating error at +40°C of 1.22°C.
PAGE 8
MAX6682 Thermistor-to-Digital Converter tCSW CS 1 2 11 1 2 SCK tDV SO tHIZ B10 MSB B9 B2 B1 B0 LSB B10 MSB B9 Figure 6. Serial Interface Timing 2 Pin Configuration Functional Diagram VCC TOP VIEW BANDGAP I.C.
PAGE 9
Thermistor-to-Digital Converter 8LUMAXD.EPS Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit patent licenses are implied. Maxim reserves the right to change the circuitry and specifications without notice at any time. Maxim Integrated Products, 120 San Gabriel Drive, Sunnyvale, CA 94086 408-737-7600 _____________________ 9 © 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.