MAX6643LBFAEE+【PDF 5/17 页】IC CTLR PWM FAN-SPEED 16QSOP

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Detailed Description

The MAX6643/MAX6644/MAX6645 measure temperature

and automatically adjust fan speed to ensure optimum

cooling while minimizing acoustic noise from the fan.

The MAX6643/MAX6644/MAX6645 generate a PWM

waveform that drives an external power transistor,

which in turn modulates the fans power supply. The

MAX6643/MAX6644/MAX6645 monitor temperature and

adjust the duty cycle of the PWM output waveform to

control the fans speed according to the cooling needs

of the system. The MAX6643 monitors its own die tem-

perature and an optional external transistors tempera-

ture, while the MAX6644 and MAX6645 each monitor

the temperatures of one or two external diode-connect-

ed transistors.

Temperature Sensor

The pn junction-based temperature sensor can mea-

sure temperatures up to two pn junctions. The

MAX6643 measures the temperature of an external

diode-connected transistor, as well as its internal tem-

perature. The MAX6644 and MAX6645 measure the

temperature of two external diode-connected transis-

tors. The temperature is measured at a rate of 1Hz.

If an external diode pin is shorted to ground or left

unconnected, the temperature is read as 0癈. Since the

larger of the two temperatures prevails, a faulty or

unconnected diode is not used for calculating fan

speed or determining overtemperature faults.

PWM Output

The larger of the two measured temperatures is always

used for fan control. The temperature is compared to

three thresholds: the high-temperature threshold (T

HIGH

the low-temperature threshold (T

LOW

), and the overtem-

perature threshold, OT. The OT comparison is done once

per second, whereas the comparisons with fan-control

thresholds T

HIGH

and T

LOW

are done once every 4s.

The duty-cycle variation of PWM_OUT from 0% to 100%

is divided into 64 steps. If the temperature measured

exceeds the threshold T

HIGH

, the PWM_OUT duty cycle

is incremented by one step, i.e., approximately 1.5%

(100/64). Similarly, if the temperature measured is below

the threshold T

LOW

, the duty cycle is decremented by

one step (1.5%). Since the T

HIGH

and T

LOW

compar-

isons are done only once every 4s, the maximum rate of

change of duty cycle is 0.4% per second.

Tables 1 and 2 show the 癈 value assigned to the TH_

and TL_ input combinations.

Automatic PWM Fan-Speed Controllers with

Overtemperature Output

_______________________________________________________________________________________ 5

Pin Description (continued)

MAX6643

MAX6644

MAX6645

NAME

FUNCTION

PWM_OUT

PWM Output for Driving External Power Transistor. Connect to

the gate of an n-channel MOSFET or to the base of an npn.

PWM_OUT requires a pullup resistor. The pullup resistor can

be connected to a supply voltage as high as 5.5V, regardless

of the supply voltage.

13, 14

OT2, OT1

Overtemperature Threshold Inputs. Connect to V

, GND, or

leave unconnected to select the upper-limit OT fault output trip

temperature, in 5癈 increments. See Table 4.

Power-Supply Input. 3.3V nominal. Bypass V

to GND with a

0.1礔 capacitor.

TH2

TH1

HIGH

(癈)

L SUFFIX

HIGH

(癈)

H SUFFIX

High-Z

Table 1. Setting T

HIGH

(MAX6643 and MAX6644)

High-Z = High impedance.

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