LTC3553 Linear Technology Corporation, LTC3553 Datasheet - Page 19

• Manufacturer Part Number: LTC3553
• Description: Micropower USB Power Manager
• Manufacturer: Linear Technology Corporation

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OPERATION
The trip points for the LTC3553’s temperature qualifi ca-
tion are internally programmed at 0.35 • V
threshold and 0.76 • V
Therefore, the hot trip point is set when:
and the cold trip point is set when:
Solving these equations for R
results in the following:
and
By setting R
in r
to the Vishay Resistance-Temperature Curve 1 chart gives
a hot trip point of about 40°C and a cold trip point of about
0°C. The difference between the hot and cold trip points
is approximately 40°C.
Figure 2. NTC Thermistor Circuit With Additional Bias Resistor
R
R
HOT
R
NTC|HOT
NTC|COLD
R
R
105k
R1
12.7k
NOM
NOM
R
100k
NOM
NTC
= 0.538 and r
R
R
NTC|HOT
15
+R
20
NTC|COLD
+R
NOM
= 0.538 • R
V
NTC
NTC|HOT
= 3.17 • R
BUS
NTC|COLD
equal to R25, the above equations result
(NTC FALLING)
(NTC FALLING)
(NTC RISING)
0.76 • V
0.35 • V
0.017 • V
COLD
• V
BUS
NOM
• V
NOM
BUS
BUS
BUS
= 3.17. Referencing these ratios
BUS
for the cold threshold.
BUS
= 0.35 • V
= 0.76 • V
–
+
–
+
+
–
NTC|COLD
BUS
BUS
BUS
and R
TOO_COLD
TOO_HOT
NTC_ENABLE
for the hot
NTC|HOT
3553 F02
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 r
sired hot and cold trip points. Note that these equations
are linked. Therefore, only one of the two trip points can
be independently set, the other is determined by the de-
fault ratios designed in the IC.
Consider an example where a 60°C hot trip point is desired.
From the Vishay curve 1 R-T characteristics, r
at 60°C. Using the above equation, R
to 46.4k. With this value of R
about 16°C. Notice that the span is now 44°C rather than
the previous 40°C. This is due to the decrease in tem-
perature gain of the thermistor as 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 2. The following formulas can
be used to compute the values of R
For example, to set the trip points to 0°C and 45°C with
a Vishay curve 1 thermistor choose:
the nearest 1% value is 105k:
The nearest 1% value is 12.7k. The fi nal solution is shown
in Figure 2 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
R1 = 0.536 • R
R
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
R
R
R
NOM
NOM
NOM
NOM
HOT
=
=
=
=
and r
r
r
0.538
3.266 – 0.4368
r
3.17
COLD
COLD
HOT
2.714
COLD
2.714
NOM
– r
• R25
• R25
HOT
are the resistance ratios at the de-
– r
• R25
HOT
NOM
• 100k =104.2k
NOM
• R25
, different in value from
, the cold trip point is
NOM
NOM
LTC3553
and R1:
should be set
HOT
is 0.2488
19
3553f