LTC3559 Linear Technology, LTC3559 Datasheet - Page 16

• Manufacturer Part Number: LTC3559
• Description: Linear USB Battery Charger
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC3559
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.
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 chosen, the other is determined by the default 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.
The upper and lower temperature trip points can be inde-
pendently programmed by using an additional bias resistor
as shown in Figure 4. The following formulas can be used
to compute the values of R
16
R
R
R
R
R
R
HOT
NTC|HOT
NTC|COLD
NOM
NOM
NOM
1 0 536
=
= 0.536 and r
HOT
=
=
=
.
r
r
0 536
and r
NOM
3 25
= 0.536 • R
COLD
COLD
r
HOT
.
= 3.25 • R
.
•
2 714
R
.
equal to R25, the above equations result
COLD
NOM
–
•
•
R
r
R
COLD
HOT
25
25
are the resistance ratios at the de-
–
NOM
NOM
r
HOT
•
= 3.25. Referencing these ratios
R
NOM
NOM
25
•
NOM
R R 25
, different in value from
and R1:
NTC|COLD
, the cold trip point is
NOM
and R
should be set
HOT
is 0.2488
NTC|HOT
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 4 and results in an upper trip point of 45°C and
a lower trip point of 0°C.
USB and Wall Adapter Power
Although the battery charger is designed to draw power
from a USB port to charge Li-Ion batteries, a wall adapter
can also be used. Figure 5 shows an example of how to
combine wall adapter and USB power inputs. A P-channel
MOSFET, MP1, is used to prevent back conduction into
the USB port when a wall adapter is present and Schottky
diode, D1, is used to prevent USB power loss through the
1k pull-down resistor.
Typically, a wall adapter can supply signifi cantly more
current than the 500mA-limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor
are used to increase the maximum charge current to
950mA when the wall adapter is present.
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
R
500mA I
950mA I
NOM
ADAPTER
5V WALL
POWER
Figure 5. Combining Wall Adapter and USB Power
USB
CHG
CHG
=
3 266 0 4368
.
MP1
2 714
.
– .
1k
D1
MN1
V
•
CHARGER
CC
BATTERY
100
1.65k
PROG
k
BAT
=
104 2
I
1.74k
BAT
.
+
k
3559 F05
Li-Ion
BATTERY
3559f