Detailed description, Pin description (continued), Table 1. setting t – Rainbow Electronics MAX6645 User Manual
Page 5
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 (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 °C value assigned to the TH_
and TL_ input combinations.
MAX6643/MAX6644/MAX6645
Automatic PWM Fan-Speed Controllers with
Overtemperature Output
_______________________________________________________________________________________
5
Pin Description (continued)
PIN
MAX6643_A MAX6643_B
MAX6644_
MAX6645_
NAME
FUNCTION
12
12
12
9
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. PWMOUT 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
13, 14
13, 14
—
OT2, OT1
Overtemperature Threshold Inputs. Connect to V
DD
, GND,
or leave unconnected to select the upper-limit OT fault
output trip temperature, in 5°C increments. See Table 4.
16
16
16
10
V
DD
Power-Supply Input. 3.3V nominal. Bypass V
DD
to GND with
a 0.1µF capacitor.
TH2
TH1
T
HIGH
(°C)
L SUFFIX
T
HIGH
(°C)
H SUFFIX
0
0
20
40
0
High-Z
25
45
0
1
30
50
High-Z
0
35
55
High-Z
High-Z
40
60
High-Z
1
45
65
1
0
50
70
1
High-Z
55
75
1
1
60
80
Table 1. Setting T
HIGH
(MAX6643 and MAX6644)
High-Z = High impedance.