MCP9902 datasheet(11/50 Pages) MICROCHIP

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 2015-2016 Microchip Technology Inc.

DS20005382C-page 11

MCP9902/3/4

4.7

Temperature Measurement

The MCP9902/3/4 can monitor the temperature of up

to three externally connected diodes.

The device contains programmable High, Low and

Therm limits for all measured temperature channels. If

the measured temperature goes below the Low limit or

above the High limit, the ALERT/THERM2 pin can be

asserted (based on user settings). If the measured

temperature meets or exceeds the Therm Limit, the

THERM pin is asserted unconditionally, providing two

tiers of temperature detection.

4.8

Beta Compensation

The MCP9902/3/4 is configured to monitor the

temperature of basic diodes (e.g., 2N3904) or CPU

thermal diodes. For the MCP9902/3/4, the External

Diode 1 channel automatically detects the type of

external diode and determines the optimal setting to

reduce temperature errors introduced by beta variation.

Compensating for this error is also known as

implementing the transistor or BJT model for

temperature measurement.

For discrete transistors configured with the collector and

base shorted together, the beta is generally sufficiently

high such that the percent change in beta variation is

very small. For example, a 10% variation in beta for two

forced emitter currents with a transistor whose ideal

beta is 50 would contribute approximately +0.25°C error

at +100°C. However, for substrate transistors where the

base-emitter junction is used for temperature measure-

ment and the collector is tied to the substrate, the pro-

portional beta variation will cause large error. For

example, a 10% variation in beta for two forced emitter

currents with a transistor whose ideal beta is 0.5 would

contribute approximately +8.25°C error at +100°C.

The MCP9904 does not support Beta Compensation on

External Diode 2 and External Diode 3 channels due to

the high beta of diode-connected transistors.

Care should be taken when setting the BETA<2:0> bits if

the auto-detection circuitry is disabled. If the Beta

Compensation factor is set at a beta value that is higher

than the transistor beta, the circuit may introduce

measurement errors. When measuring a discrete

thermal diode (such as 2N3904) or a CPU diode that

functions like a discrete thermal diode (such as an AMD

processor diode), the BETA<2:0> bits should be set to

‘111b’.

4.9

Resistance Error Correction (REC)

Parasitic resistance in series with the external diodes

will limit the accuracy obtainable from temperature

measurement devices. The voltage developed across

this resistance by the switching diode currents causes

the temperature measurement to read higher than the

true temperature. Contributors to series resistance are

PCB trace resistance, on die (i.e., on the processor)

metal resistance, bulk resistance in the base and emit-

ter of the temperature transistor. Typically, the error

caused by series resistance is +0.7°C per ohm. The

MCP9902/3/4 automatically corrects up to 100 ohms

of series resistance.

4.10

Programmable External Diode

Ideality Factor

The MCP9902/3/4 is designed for external diodes with

an ideality factor of 1.008. Not all external diodes,

processor or discrete, will have this exact value. This

variation of the ideality factor introduces temperature

measurement errors which must be corrected. This

correction is typically done using programmable offset

registers. Since an ideality factor mismatch introduces

an error that is a function of temperature, this correction

is only accurate within a small range of temperatures.

To provide maximum flexibility to the user, the

MCP9902/3/4 provides a 6-bit register for each external

diode where the ideality factor of the diode used is

programmed

eliminate

errors

across

all

temperatures.

These registers store the ideality factors that are

applied to the external diode. Table 4-3 defines each

setting and the corresponding ideality factor. Beta

Compensation and Resistance Error Correction

automatically correct for most diode ideality errors;

therefore, it is not recommended that these settings be

updated without consulting Microchip Technology Inc.

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