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6 Detailed Description

6.1 Overview

The LM64 Remote Diode Temperature Sensor with Integrated Fan Control incorporates a ΔVBE-based

temperature sensor using a Local or Remote diode and a 10-bit plus sign ΔΣ ADC (Delta-Sigma Analog-to-

Digital Converter). The pulse-width modulated (PWM) open-drain output, with a pull-up resistor, can drive a

switching transistor to modulate the fan. The LM64 can measure the fan speed on the pulses from the fan’s

open-collector tachometer output, pulled up by a 1.5 kΩ resistor to VDD. The ALERT open-drain output will be

pulled low under certain conditions descibed in the sections below. The T_Crit open-drain output will be pulled

low when the T_Crit setpoint temperature limit is exceeded. This behaves as a typical comparator function

without any latching.

The LM64's two-wire interface is compatible with the SMBus Specification 2.0 . For more information the reader

is directed to www.smbus.org.

In the LM64, digital comparators are used to compare the measured Local Temperature (LT) to the Local

High Setpoint user-programmable temperature limit register. The measured Remote Temperature (RT) is

digitally compared to the Remote High Setpoint (RHS), the Remote Low Setpoint (RLS), and the Remote

T_CRIT Setpoint (RCS) user-programmable temperature limits. An ALERT output will occur when the measured

temperature is: (1) higher than either the High Setpoint or the T_CRIT Setpoint, or (2) lower than the Low

Setpoint. The ALERT Mask register allows the user to prevent the generation of these ALERT outputs.

The temperature hysteresis is set by the value placed in the Hysteresis Register (TH).

The LM64 may be placed in a low power Standby mode by setting the Standby bit found in the Configuration

to allow the PWM output signal to continue, or not, by programming the PWM Disable in Standby bit in the

Configuration Register.

The Local Temperature reading and setpoint data registers are 8-bits wide. The format of the 11-bit remote

temperature data is a 16-bit left justified word. Two 8-bit registers, high and low bytes, are provided for each

setpoint as well as the temperature reading. Two Remote Temperature Offset (RTO) Registers: High Byte and

Low Byte (RTOHB and RTOLB) may be used to correct the temperature readings by adding or subtracting a

fixed value based on a different non-ideality factor of the thermal diode if different from the graphics processor

thermal diode. See Section 8.1.3.1.

LM64

SNAS207B – MAY 2004 – REVISED JANUARY 2024

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