Hysteresis Input

The HYST pin is a CMOS-compatible input that selects

or a low level (5°C for HYST = GND). Hysteresis pre-

vents the output from chattering when the temperature

is near the trip point. The HYST pin must not float.

The output asserts when the temperature exceeds the

trip point and deasserts when the temperature falls

back below the trip point minus the hysteresis. For

example, if the trip point is 105°C, the output will assert

at 105°C and will not deassert until temperature falls

below 105°C minus the hysteresis (e.g., 95°C if 10°C

hysteresis is chosen) (Figure 2).

Applications Information

Remote-Diode Selection

To ensure best accuracy, use a good-quality diode-

connected transistor. Suggested devices are listed in

Table 1. Large power transistors are not recommend-

ed. Tight specifications for forward current gain indi-

cate the manufacturer has good process controls and

The MAX6511/MAX6512/MAX6513 can also measure

the die temperature of CPUs and other integrated cir-

cuits having on-board temperature-sensing diodes.

Use the monitor’s output to reset the µP, assert an inter-

rupt, activate a cooling fan, or trigger an external alarm.

Noise Filtering Capacitors

A quality ceramic capacitor must be connected across

the DXP/DXN inputs to maintain temperature threshold

accuracy by filtering out noise. The capacitor should be

located physically close to the DXP/DXN pins and

should typically have a value of 2200pF. Larger capaci-

tor values can cause temperature measurement errors.

A 50% variation from the recommended capacitor

value can cause up to ±1°C error.

Low-Cost, Remote SOT Temperature Switches

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5

Figure 2. Temperature Trip Threshold Hysteresis

TRIP TEMPERATURE

TRIP TEMPERATURE HYSTERESIS

TOVER (MAX6511)

TIME

MANUFACTURER

MODEL NUMBER

Central Semiconductor (USA)

CMPT3904

ON (USA)

MMBT3904

Rohm Semiconductor (Japan)

SST3904

Samsung (Korea)

KST3904-TF

Siemens (Germany)

SMBT3904

Zetex (England)

FMMT3904CT-ND

Table 1. Sensor Transistor Manufacturers

Note: Transistors must be diode connected (base shorted to

collector).