LTC3557-1 Linear Technology, LTC3557-1 Datasheet - Page 24

• Manufacturer Part Number: LTC3557-1
• Description: USB Power Manager
• Manufacturer: Linear Technology

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LTC3557/LTC3557-1
APPLICATIONS INFORMATION
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 inde-
pendently programmed by using an additional bias resistor
as shown in Figure 9. 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.
24
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
R
R
R
NOM
1 0 536
NOM
=
R
100k
R
100k
NOM
V
NTC
NTC
18
19
NTC
=
.
=
Figure 8. Typical NTC Thermistor Circuit
r
3 266 0 4368
COLD
.
•
2 714
R
0.017 • V
0.349 • V
.
2 714
0.765 • V
NOM
–
.
– .
r
HOT
VNTC
VNTC
–
VNTC
r
HOT
•
R
NOM
25
•
•
NTC BLOCK
–
+
–
+
+
–
100
NOM
R R 25
and R1:
k
, the cold trip point is
=
104 2
NOM
.
TOO_COLD
TOO_HOT
NTC_ENABLE
should be set
k
HOT
is 0.2488
35571 F08
the nearest 1% value is 12.7k. The fi nal solution is shown
in Figure 9 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
Battery Charger Stability Considerations
The LTC3557/LTC3557-1’s battery charger contains both
a constant voltage and a constant current control loop.
The constant voltage loop is stable without any compen-
sation when a battery is connected with low impedance
leads. Excessive lead length, however, may add enough
series inductance to require a bypass capacitor of at least
1μF from BAT to GND. Furthermore, a 4.7μF capacitor in
series with a 0.2Ω to 1Ω resistor from BAT to GND is
required to keep ripple voltage low when the battery is
disconnected.
High value, low ESR multilayer ceramic chip capacitors
reduce the constant voltage loop phase margin, possibly
resulting in instability. Ceramic capacitors up to 22μF may
be used in parallel with a battery, but larger ceramics should
be decoupled with 0.2Ω to 1Ω of series resistance.
In constant current mode, the PROG pin is in the feed-
back loop rather than the battery voltage. Because of the
additional pole created by any PROG pin capacitance,
Figure 9. NTC Thermistor Circuit with Additional Bias Resistor
R
105k
R1
12.7k
R
100k
NOM
V
NTC
NTC
18
19
NTC
0.017 • V
0.349 • V
0.765 • V
VNTC
VNTC
VNTC
NTC BLOCK
–
+
–
+
+
–
TOO_COLD
TOO_HOT
NTC_ENABLE
35571 F09
35571f