MAX6643LBFAEE+【PDF 12/17 页】IC CTLR PWM FAN-SPEED 16QSOP

Automatic PWM Fan-Speed Controllers with

Overtemperature Output

12 ______________________________________________________________________________________

Effect of Series Resistance

Series resistance in a sense diode contributes addition-

al errors. For nominal diode currents of 10礎 and

100礎, change in the measured voltage is:

Since 1癈 corresponds to 198.6礦, series resistance

contributes a temperature offset of:

Assume that the diode being measured has a series

resistance of 3? The series resistance contributes an

offset of:

The effects of the ideality factor and series resistance

are additive. If the diode has an ideality factor of 1.008

and series resistance of 3? the total offset can be cal-

culated by adding error due to series resistance with

error due to ideality factor:

1.36癈 - 0.66癈 = 0.7癈

for a diode temperature of +60.7癈.

In this example, the effect of the series resistance and

the ideality factor partially cancel each other.

For best accuracy, the discrete transistor should be a

small-signal device with its collector connected to

base, and emitter connected to GND. Table 5 lists

examples of discrete transistors that are appropriate for

use with the MAX6643/MAX6644/MAX6645.

The transistor must have a relatively high forward volt-

age; otherwise, the ADC input voltage range can be vio-

lated. The forward voltage at the highest expected

temperature must be greater than 0.25V at 10礎, and at

the lowest expected temperature, the forward voltage

must be less than 0.95V at 100礎. Large power transis-

tors must not be used. Also, ensure that the base resis-

tance is less than 100? Tight specifications for forward

current gain (50 < ?<150, for example) indicate that the

manufacturer has good process controls and that the

devices have consistent V

BE

characteristics.

ADC Noise Filtering

The integrating ADC has inherently good noise rejec-

tion, especially of low-frequency signals such as

60Hz/120Hz power-supply hum. Micropower operation

places constraints on high-frequency noise rejection.

Lay out the PCB carefully with proper external noise fil-

tering for high-accuracy remote measurements in elec-

trically noisy environments.

Filter high-frequency electromagnetic interference

(EMI) at the DXP pins with an external 2200pF capaci-

tor connected between DXP, DXP1, or DXP2 and

ground. This capacitor can be increased to about

3300pF (max), including cable capacitance. A capaci-

tance higher than 3300pF introduces errors due to the

rise time of the switched-current source.

Twisted Pairs and Shielded Cables

For remote-sensor distances longer than 8in, or in par-

ticularly noisy environments, a twisted pair is recom-

mended. Its practical length is 6ft to 12ft (typ) before

noise becomes a problem, as tested in a noisy electron-

ics laboratory. For longer distances, the best solution is

a shielded twisted pair like that used for audio micro-

phones. For example, Belden 8451 works well for dis-

tances up to 100ft in a noisy environment. Connect the

twisted pair to DXP and GND and the shield to ground,

and leave the shields remote end unterminated. Excess

capacitance at DXP limits practical remote-sensor dis-

tances (see the Typical Operating Characteristics).

For very long cable runs, the cables parasitic capaci-

tance often provides noise filtering, so the recommend-

ed 2200pF capacitor can often be removed or reduced

in value. Cable resistance also affects remote-sensor

accuracy. A 1?series resistance introduces about

+1/2癈 error.

PCB Layout Checklist

1) Place the MAX6643/MAX6644/MAX6645 as close as

practical to the remote diode. In a noisy environment,

such as a computer motherboard, this distance can

be 4in to 8in or more, as long as the worst noise

sources (such as CRTs, clock generators, memory

buses, and ISA/PCI buses) are avoided.

2) Do not route the DXP lines next to the deflection coils

of a CRT. Also, do not route the traces across a fast

memory bus, which can easily introduce +30癈 error,

even with good filtering. Otherwise, most noise

sources are fairly benign.

3

C

?/DIV>

=

?/DIV>

.

0 453

1 36

90

198 6

0 453

?/DIV>

=

?/DIV>

V

C

?/DIV>

.

擵

M

=

?nbsp ?/DIV>

(

)

= ?nbsp ?/DIV>

R

A

A R

S

s

100

10

90

发布紧急采购，3分钟左右您将得到回复。

采购需求

(若只采购一条型号，填写一行即可)

*型号	*数量	厂商	批号	封装

添加更多采购

我的联系方式

*

采购需求

发布成功！您可以继续发布采购。也可以进入我的后台，查看报价

发布成功！您可以继续发布采购。也可以进入我的后台，查看报价

我的联系方式