f75395 Feature Integration Technology Inc., f75395 Datasheet - Page 12

• Manufacturer Part Number: f75395
• Description: ?1o C Temperature Sensor With ? Compensation
• Manufacturer: Feature Integration Technology Inc.

6.11 Resistor Cancelled Function
6.12 PCB Layout Guide
F75395
The F75395 measures temperature from the thermal diodes by the basic. In traditional case, the F75395 outputs dual currents to a
thermal diode. Then the F75395 calculates the absolute temperature by △ V
normally very high such that the percent change in beta is very small. For example, 15% variation in beta for two forced I
and the beta is 50 would contribute about 0.32 ℃ error per 100 ℃ . For Substrate PNP transistor (i.e. CPU), the beta is very small
such that the proportional beta variation will very high, and it will cause large error in temperature measurement. For example, 15%
variation in beta for two forced I
In Order to solve the second issue, the F75395 provides a beta compensation solution for accurate temperature sensing. There is a
register (CR30h bit7) for external thermal diode selection by Beta variation. If this bit is enabled, the beta compensation will
automate to measure the temperature from substrate transistor (i.e. CPU). The F75395 can support the beta range from 0.05~1.8 for
beta compensation. In this new method, the F75395 will provide two I
auto-adjust I
accurate temperature. This algorithm of beta compensation is suitable for substrate transistor or new generational CPU (i.e. 45nm
CPU) because small beta and high proportional beta variation. The default value of register CR3Bh bit7 is enabled for measure
substrate transistor. If user would like to detect discrete transistor and the beta is big enough, please disable this bit for detecting.
About this section application, please refer the register description for detail.
The F75393/F75394 can cancel resistor effect from CPU internal circuit or PCB circuit.
PCB can be electrically noisy environments, and the F75395 is measuring very small voltage from the remote sensor, so care must
be taken to minimize noise which is occurred at the sensor inputs. The following guideline should be taken to reduce the
measurement error of the temperature sensors:
1.
2.
Place the F75395 as close as practical to the remote sensing diode. In noisy environments, such as a computer main-board,
the distance can be 4 to 8 inches. (typ). This length can be increased if the worst noise sources are avoided. Noise sources
generally include clock generators, CRTs, memory buses and PCI/ISA bus etc.
Route the D+ and D- tracks close together, in parallel, with grounded guard tracks on each side. Provide a ground plane under
the tracks if possible. Do not route D+ & D- lines next to the deflection coil of the CRT. And also don’t route the trace across fast
E
(I
E1
and I
E2
) current and feedback I
∆
E
V
currents and the beta is 0.5 would contribute about 11.12 ℃ error per 100 ℃ .
BE
=
KT
q
×
ln
B
(I
Ie
B1
Ie
1
2
and I
=
-8-
B2
KT
q
) promptly for getting proper I
×
ln
 +
 

 +
 

1
E
1
currents, and feedback two I
β
β
β
2
1
β
BE
2
1
. For discrete transistor (i.e. 2N3906), the beta is

 


 

Ic
Ic
1
2
C
proportion (I
B
C1
currents. The F75395 will
/ I
C2
F75395
), then calculates the
May, 2008
E
V0.15P
currents