LTC4095 Datasheet, PDF (12/16 Pages) Linear Technology – Standalone USB Li-Ion/Polymer Battery Charger in 2mm × 2mm DFN

LTC4095
APPLICATIONS INFORMATION
Solving these equations for R_NTC|COLD and R_NTC|HOT
results in the following:
R_NTC|HOT = 0.536 • R_NOM
and
R_NTC|COLD = 3.25 • R_NOM
By setting R_NOM equal to R25, the above equations result
in r_HOT = 0.536 and r_COLD = 3.25. Referencing these ratios
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_NOM, different in value from
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:
R_NOM
=
r_HOT
0.536
•
R25
R_NOM
=
r_COLD
3.25
• R25
where r_HOT and r_COLD are the resistance ratios at the de-
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_HOT is 0.2488
at 60°C. Using the above equation, R_NOM should be set
to 46.4k. With this value of R_NOM, the cold trip point is
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_NOM and R1:
R_NOM
=
r_COLD – r_HOT
2.714
• R25
R1= 0.536 • R_NOM – r_HOT • R25
12
For example, to set the trip points to 0°C and 45°C with
a Vishay Curve 1 thermistor choose:
R_NOM
=
3.266 – 0.4368
2.714
•
100k
=
104.2k
the nearest 1% value is 105k.
R1 = 0.536 • 105k – 0.4368 • 100k = 12.6k
the nearest 1% value is 12.7k. The final 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 LTC4095 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 significantly 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.
5V WALL
ADAPTER
950mA I_CHG
USB
POWER
500mA I_CHG
I_BAT
1
BAT
D1
LTC4095
8
IN
MP1
7
PROG
+ Li-Ion
BATTERY
MN1 ^1.65k
1k
1.74k
4095 F05
Figure 5. Combining Wall Adapter and USB Power
Power Dissipation
The conditions that cause the LTC4095 to reduce charge
current through thermal feedback can be approximated
by considering the power dissipated in the IC. For high
4095fa