LTC4090-5_15 Datasheet, PDF (24/30 Pages) Linear Technology – USB Power Manager with 2A High Voltage Bat-Track Buck Regulator

LTC4090/LTC4090-5
APPLICATIONS INFORMATION
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
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:
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
previous 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 final 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
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 efficiency measurement, and subtract-
ing the catch diode loss.
[ ] ^P_D ^{= (1− h)• V}_HVOUT ^•(I_BAT ^+I_OUT ⁾
− V_D
• ^{}
1−
V_HVOUT
V_HVIN

• (I_BAT
+I_OUT )+ 0.3V •I_BAT )
where h is the efficiency of the high voltage regulator and
V_D is the forward voltage of the catch diode at I = I_BAT
+ I_OUT. The first 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% efficiency, 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 be estimated
4090fd
24
For more information www.linear.com/LTC4090