MAX6616AEG+T Datasheet MAX6615AEE+, MAX6615AEE+T, MAX6616AEG+T | P7-P9

(POR) state. See Tables 3–7 for all other register functions

and the

section.

Temperature Measurements

The averaging ADC integrates over a 120ms period

(each channel, typically), with excellent noise rejection.

For internal temperature measurements, the ADC and

associated circuitry measure the forward voltage of the

internal sensing diode at low- and high-current levels

and compute the temperature based on this voltage.

For thermistor measurements, the reference voltage

and the thermistor voltage are measured and offset is

applied to yield a value that correlates well to thermistor

temperature within a wide temperature range. Both

channels are automatically converted once the conver-

sion process has started. If one of the two channels is

not used, the circuit still performs both measurements,

and the data from the unused channel may be ignored.

If either of the measured temperature values is below

0°, the value in the corresponding temperature register

is clipped to zero when a negative offset is pro-

grammed into the thermistor offset register (17h).

Local (internal) temperature data is expressed directly

in degrees Celsius. Two registers contain the tempera-

ture data for the local channel. The high-byte register

has an MSB of 128°C and an LSB of 1°C. The low- byte

LSB of 0.125°C. The data format is shown in Table 1.

Thermistors allow measurements of external tempera-

tures. Connect a thermistor in series with a resistor,

EXT

. The thermistor should be connected between the

TH_ input and ground, and R

EXT

should be connected

between the reference output, REF, and the TH_ input,

as shown in the

Typical Application Circuit

The voltage across R

EXT

is measured by the ADC,

resulting in a value that is directly related to tempera-

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

_______________________________________________________________________________________ 7

SMBCLK

A = START CONDITION

B = MSB OF ADDRESS CLOCKED INTO SLAVE

C = LSB OF ADDRESS CLOCKED INTO SLAVE

D = R/W BIT CLOCKED INTO SLAVE

AB CD

HIJ

SMBDATA

SU:STA

HD:STA

LOW

HIGH

SU:DAT

SU:STO

BUF

LMK

E = SLAVE PULLS SMBDATA LINE LOW

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER

G = MSB OF DATA CLOCKED INTO SLAVE

H = LSB OF DATA CLOCKED INTO SLAVE

I = MASTER PULLS DATA LINE LOW

J = ACKNOWLEDGE CLOCKED INTO SLAVE

K = ACKNOWLEDGE CLOCK PULSE

L = STOP CONDITION

M = NEW START CONDITION

Figure 2. SMBus Write Timing Diagram

SMBCLK

AB CD

FG H

SMBDATA

SU:STA

HD:STA

LOW

HIGH

SU:DAT

HD:DAT

SU:STO

BUF

A = START CONDITION

B = MSB OF ADDRESS CLOCKED INTO SLAVE

C = LSB OF ADDRESS CLOCKED INTO SLAVE

D = R/W BIT CLOCKED INTO SLAVE

E = SLAVE PULLS SMBDATA LINE LOW

F = ACKNOWLEDGE BIT CLOCKED INTO MASTER

G = MSB OF DATA CLOCKED INTO MASTER

H = LSB OF DATA CLOCKED INTO MASTER

I = MASTER PULLS DATA LINE LOW

J = ACKNOWLEDGE CLOCKED INTO SLAVE

K = ACKNOWLEDGE CLOCK PULSE

L = STOP CONDITION

M = NEW START CONDITION

Figure 3. SMBus Read Timing Diagram

MAX6615/MAX6616

ture. The thermistor data in the temperature register(s)

gives the voltage across R

EXT

as a fraction of the refer-

ence voltage. The LSB of the high byte has a nominal

weight of 7.68mV.

OOTT

Output

The OT output asserts when a thermal fault occurs, and

can therefore be used as a warning flag to initiate sys-

tem shutdown, or to throttle clock frequency. When

temperature exceeds the OT temperature threshold

and OT is not masked, the OT status register indicates

a fault and OT output becomes asserted. If OT for the

respective channel is masked off, the OT status register

continues to be set, but the OT output does not

become asserted.

The fault flag and the output can be cleared by reading

the OT status register. The OT output can also be

cleared by masking the affected channel. If the OT sta-

tus bit is cleared, OT reasserts on the next conversion if

the temperature still exceeds the OT temperature

threshold.

PWM Output

The PWM_ signals are normally used in one of three

ways to control the fan’s speed:

1) PWM_ drives the gate of a MOSFET or the base of a

bipolar transistor in series with the fan’s power sup-

ply. The

Typical Application Circuit

shows the PWM_

driving an n-channel MOSFET. In this case, the PWM

invert bit (D4 in register 02h) is set to 1. Figure 4

shows PWM_ driving a p-channel MOSFET and the

PWM invert bit must be set to zero.

2) PWM_ is converted (using an external circuit) into a

DC voltage that is proportional to duty cycle. This

duty-cycle-controlled voltage becomes the power

supply for the fan. This approach is less efficient

than (1), but can result in quieter fan operation.

Figure 5 shows an example of a circuit that converts

the PWM signal to a DC voltage. Because this circuit

produces a full-scale output voltage when PWM =

0V, bit D4 in register 02h should be set to zero.

3) PWM_ directly drives the logic-level PWM speed-

control input on a fan that has this type of input. This

approach requires fewer external components and

combines the efficiency of (1) with the low noise of

(2). An example of PWM_ driving a fan with a speed-

control input is shown in Figure 6. Bit D4 in register

02h should be set to 1 when this configuration is

used.

Whenever the fan has to start turning from a motionless

state, PWM_ is forced high for 2s. After this spin-up

period, the PWM_ duty cycle settles to the predeter-

mined value. Whenever spin-up is disabled (bit 2 in the

configuration byte = 1) and the fan is off, the duty cycle

changes immediately from zero to the nominal value,

ignoring the duty-cycle rate-of-change setting.

The frequency-select register controls the frequency of

the PWM signal. When the PWM signal modulates the

power supply of the fan, a low PWM frequency (usually

33Hz) should be used to ensure the circuitry of the

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

8 _______________________________________________________________________________________

Table 1. Temperature Data Format (High Byte and Low Byte)

HIGH BYTE LOW BYTE

TEMPERATURE (°C)

BINARY VALUE HEX VALUE BINARY VALUE HEX VALUE

140.0 1000 1100 8Ch 0000 0000 00h

127.0 0111 1111 7Fh 0000 0000 00h

25.375 0001 1001 19h 0110 0000 60h

25.0 0001 1001 19h 0000 0000 00h

0.5 0000 0000 00h 1000 0000 80h

0.0 0000 0000 00h 0000 0000 00h

<0 0000 0000 00h 0000 0000 00h

PWM

10kΩ

Figure 4. Driving a p-Channel MOSFET for Top-Side PWM Fan

Drive

brushless DC motor has enough time to operate. When

driving a fan with a PWM-to-DC circuit as shown in

Figure 5, the highest available frequency (35kHz) should

be used to minimize the size of the filter capacitors.

When using a fan with a PWM control input, the frequen-

cy normally should be high as well, although some fans

have PWM inputs that accept low-frequency drive.

The duty cycle of the PWM can be controlled in two ways:

1) Manual PWM control: setting the duty cycle of the fan

directly through the fan target duty-cycle registers

(0Bh and 0Ch).

2) Automatic PWM control: setting the duty cycle based

on temperature.

Manual PWM Duty-Cycle Control

Clearing the bits that select the temperature channels for

fan control (D5 and D4 for PWM1 and D3 and D2 for

PWM2) in the fan-configuration register (11h) enables

manual fan control. In this mode, the duty cycle written to

the fan target duty-cycle register directly controls the

corresponding fan. The value is clipped to a maximum of

240. Any value entered above that is changed to 240

automatically. In this control mode, the value in the maxi-

mum duty-cycle register is ignored and does not affect

the duty cycle used to control the fan.

Automatic PWM Duty-Cycle Control

In the automatic control mode, the duty cycle is con-

trolled by the local or remote temperature according to

the settings in the control registers. Below the fan-start

temperature, the duty cycle is either 0% or is equal to

the fan-start duty cycle, depending on the value of bit

D3 in the configuration byte register. Above the fan-

start temperature, the duty cycle increases by one

duty-cycle step each time the temperature increases by

one temperature step. The target duty cycle is calculat-

ed based on the following formula; for temperature >

FanStartTemperature:

where:

DC = DutyCycle

FSDC = FanStartDutyCycle

T = Temperature

FST = FanStartTemperature

DCSS = DutyCycleStepSize

TS = TempStep

Duty cycle is recalculated after each temperature con-

version if temperature is increasing. If the temperature

begins to decrease, the duty cycle is not recalculated

until the temperature drops by 5°C from the last peak

temperature. The duty cycle remains the same until the

temperature drops 5°C from the last peak temperature or

the temperature rises above the last peak temperature.

For example, if the temperature goes up to +85°C and

starts decreasing, duty cycle is not recalculated until the

temperature reaches +80°C or the temperature rises

above +85°C. If the temperature decreases further, the

duty cycle is not updated until it reaches +75°C.

For temperature < FanStartTemperature and D2 of

configuration register = 0:

DutyCycle = 0

For temperature < FanStartTemperature and D2 of

configuration register = 1:

DutyCycle = FanStartDutyCycle

Once the temperature crosses the fan-start temperature

threshold, the temperature has to drop below the fan-

start temperature threshold minus the hysteresis before

DC FSDC T FST

DCSS

=+ × ( ) -

MAX6615/MAX6616

Dual-Channel Temperature Monitors and

Fan-Speed Controllers with Thermistor Inputs

_______________________________________________________________________________________ 9

+3.3V

PWM

18kΩ

27kΩ

10kΩ 120kΩ

+3.3V

+12V

500kΩ

OUT

TO FAN

1µF

0.01µF

0.1µF

Figure 5. Driving a Fan with a PWM-to-DC Circuit

PWM

4.7kΩ

Figure 6. Controlling a PWM Input Fan with the MAX6615/

MAX6616s’ PWM Output (Typically, the 35kHz PWM

Frequency Is Used)

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P20

MAX6616AEG+T

Mfr. #:

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Manufacturer:

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Description:

Board Mount Temperature Sensors Dual Ch Temperature Monitor

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MAX6616AEG+T

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