MAX6645 데이터 시트보기 (PDF) - Maxim Integrated
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MAX6645 Datasheet PDF : 17 Pages
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Automatic PWM Fan-Speed Controllers with
Overtemperature Output
MAX6643
12
13, 14
16
PIN
MAX6644
12
13, 14
16
MAX6645
9
—
10
Pin Description (continued)
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.
OT2, OT1
Overtemperature Threshold Inputs. Connect to VDD, GND, or
leave unconnected to select the upper-limit OT fault output trip
temperature, in 5°C increments. See Table 4.
VDD
Power-Supply Input. 3.3V nominal. Bypass VDD to GND with a
0.1µF capacitor.
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 fan’s power supply. The
MAX6643/MAX6644/MAX6645 monitor temperature and
adjust the duty cycle of the PWM output waveform to
control the fan’s speed according to the cooling needs
of the system. The MAX6643 monitors its own die tem-
perature and an optional external transistor’s 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°C. 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 (THIGH),
the low-temperature threshold (TLOW), and the overtem-
perature threshold, OT. The OT comparison is done once
per second, whereas the comparisons with fan-control
thresholds THIGH and TLOW 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 THIGH, 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 TLOW, the duty cycle is decremented by
one step (1.5%). Since the THIGH and TLOW 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 °C value assigned to the TH_
and TL_ input combinations.
Table 1. Setting THIGH
(MAX6643 and MAX6644)
TH2
TH1
THIGH (°C)
L SUFFIX
0
0
20
0
High-Z
25
0
1
30
High-Z
0
35
High-Z High-Z
40
High-Z
1
45
1
0
50
1
High-Z
55
1
1
60
High-Z = High impedance.
THIGH (°C)
H SUFFIX
40
45
50
55
60
65
70
75
80
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