Datasheet

LTC4090/LTC4090-5
24
4090fc
equations can be used to easily calculate a new value for
the bias resistor:
R
NOM
=
r
HOT
0.409
•R
25C
R
NOM
=
r
COLD
2.815
•R
25C
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 40°C
hot trip point is desired.
From the Vishay curve 2 R-T characteristics, r
HOT
is 0.5758
at 40°C. Using the above equation, R
NOM
should be set to
14.0k. With this value of R
NOM
, the cold trip point is about
–7°C. Notice that the span is now 47°C rather than the pre-
vious 50°C. This is due to the increase in temperature gain
of the thermistor as absolute temperature decreases.
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
NOM
and R1:
R
NOM
=
r
COLD
–r
HOT
2.815
•R
25C
R1= 0.409 •R
NOM
–r
HOT
•R
25C
For example, to set the trip points to –5°C and 55°C with
a Vishay curve 2 thermistor choose
R
NOM
=
3.532 – 0.3467
2.815 – 0.409
•10k=13.2k
the nearest 1% value is 13.3k.
R1 = 0.409 • 13.3k – 0.3467 • 10k = 1.97k
the nearest 1% value is 1.96k. The fi nal solution is shown
in Figure 9 and results in an upper trip point of 55°C and
a lower trip point of –5°C.
Power Dissipation and High Temperature Considerations
The die temperature of the LTC4090/LTC4090-5 must be
lower than the maximum rating of 110°C. This is generally
not a concern unless the ambient temperature is above
85°C. The total power dissipated inside the LTC4090/
LTC4090-5 depend on many factors, including input voltage
(IN or HVIN), battery voltage, programmed charge current,
programmed input current limit, and load current.
In general, if the LTC4090/LTC4090-5 is being powered from
IN the power dissipation can be calculated as follows:
P
D
= (V
IN
– V
BAT
) • I
BAT
+ (V
IN
– V
OUT
) • I
OUT
where P
D
is the power dissipated, I
BAT
is the battery
charge current, and I
OUT
is the application load current.
For a typical application, an example of this calculation
would be:
P
D
= (5V – 3.7V) • 0.4A + (5V – 4.75V) • 0.1A
= 545mW
This examples assumes V
IN
= 5V, V
OUT
= 4.75V, V
BAT
=
3.7V, I
BAT
= 400mA, and I
OUT
= 100mA resulting in slightly
more than 0.5W total dissipation.
If the LTC4090 is being powered from HVIN, the power
dissipation can be estimated by calculating the regulator
power loss from an effi ciency measurement, and subtract-
ing the catch diode loss.
P
D
=(1−η)• V
HVOUT
•(I
BAT
+I
OUT
)
⎡
⎣
⎤
⎦
−V
D
•1−
V
HVOUT
V
HVIN
⎛
⎝
⎜
⎞
⎠
⎟
•I
BAT
+I
OUT
)+ 0.3V •I
BAT
()
where η is the effi ciency of the high voltage regulator and
V
D
is the forward voltage of the catch diode at I = I
BAT
+ I
OUT
. The fi rst term corresponds to the power lost in
converting V
HVIN
to V
HVOUT
, the second term subtracts
the catch diode loss, and the third term is the power dis-
sipated in the battery charger. For a typical application, an
example of this calculation would be:
P
D
=(1− 0.87)• 4V •(1A + 0.6A)
[]
−0.4V • 1−
4V
12V
⎛
⎝
⎜
⎞
⎠
⎟
•1A+ 0.6A
()
+ 0.3V • 1A = 0.7W
This example assumes 87% effi ciency, V
HVIN
= 12V, V
BAT
= 3.7V (V
HVOUT
is about 4V), I
BAT
= 1A, I
OUT
= 600mA
resulting in about 0.7W total dissipation. If the LTC4090-5
is being powered from HVIN, the power dissipation can
APPLICATIONS INFORMATION