LTC4099 Linear Technology Corporation, LTC4099 Datasheet - Page 29

• Manufacturer Part Number: LTC4099
• Description: I2C Controlled USB Power Manager/Charger
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
By using a bias resistor, R
R25, the hot and cold trip points can be moved in either
direction. The temperature span will change somewhat due
to the nonlinear behavior of the thermistor. The following
equations can be used to easily calculate a new value for
the bias resistor:
where
desired hot and cold trip points. Note that these equations
are linked. Therefore, only one of the two trip points can
be chosen; the other is determined by the default ratios
designed in the LTC4099. Consider an example where a
50°C hot trip point is desired.
From the Vishay curve 2 R-T characteristics,
0.4086 at 50°C. Using the prior equation, R
be set to 8.45k. With this value of R
and the cold trip point is about 4°C. Notice that the span
is now 46°C rather than the previous 45°C. This is due to
R
R
NOM
NOM
T
R
10k
NOM
NTCBIAS
R
10k
NTC
HOT
NTC
0 4839
3
2
Figure 7. Standard NTC Confi guration
2 816
and
.
.
COLD
HOT
0.326 • NTCBIAS
0.738 • NTCBIAS
0.228 • NTCBIAS
COLD
•
•
R
R
25
25
are the resistance ratios at the
NOM
–
+
–
+
+
–
NTC BLOCK
LTC4099
, different in value from
NOM
,
COLD
TOO_COLD
TOO_HOT
OVERTEMP
NOM
is 2.380
should
HOT
4099 F07
is
the decrease in the temperature gain of the thermistor as
the absolute temperature increases.
The upper and lower temperature trip points can be in-
dependently programmed by using an additional bias
resistor as shown in Figure 8. The following formulas can
be used to compute the values of R
For example, to set the trip points to 0°C and 50°C with
a Vishay curve 2 thermistor, choose:
the nearest 1% value is 10.2k:
The nearest 1% value is 845Ω. The fi nal circuit is shown
in Figure 8, and results in an upper trip point of 50°C and
a lower trip point of 0°C.
R1 = 0.4839 • 10.2k – 0.4086 • 10k = 0.850k
R
R
R
T
NOM
1 0 4839
NOM
R
10.2k
R1
845Ω
NOM
NTCBIAS
R
10k
NTC
NTC
.
3
2
2 816 0 4086
Figure 8. Modifi ed NTC Confi guration
.
COLD
0.326 • NTCBIAS
•
0.228 • NTCBIAS
2 332
0.738 • NTCBIAS
2 332
R
.
.
– .
NOM
–
HOT
–
•
HOT
•
R
–
+
–
+
+
–
10
NTC BLOCK
25
LTC4099
• • R25
k
NOM
10 32
.
LTC4099
and R1:
k
TOO_COLD
TOO_HOT
OVERTEMP
29
4099 F08
4099f