LTC4090EDJC(RevA) View Datasheet(PDF) - Linear Technology

Part Name

Description

MFG CO.

LTC4090EDJC
(Rev.:RevA)

USB Power Manager with 2A High Voltage Bat-Track Buck Regulator

Linear Technology 

LTC4090/LTC4090-5

APPLICATIONS INFORMATION

where rHOT and rCOLD 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, rHOT is 0.5758

at 40°C. Using the above equation, RNOM should be set

to 14.0k. With this value of RNOM, 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 “tem-

perature 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 RNOM and R1:

RNOM

=

rCOLD – rHOT

2.815

• R25C

R1= 0.409 •RNOM – rHOT •R25C

For example, to set the trip points to -5°C and 55°C with

a Vishay Curve 2 thermistor choose

RNOM

=

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

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:

PD = (VIN – VBAT) • IBAT + (VIN – VOUT) • IOUT

where PD is the power dissipated, IBAT is the battery

charge current, and IOUT is the application load current.

For a typical application, an example of this calculation

would be:

PD = (5V – 3.7V) • 0.4A + (5V – 4.75V) • 0.1A

= 545mW

This examples assumes VIN = 5V, VOUT = 4.75V, VBAT =

3.7V, IBAT = 400mA, and IOUT = 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.

PD = (1− η) • ⎡⎣VHVOUT •(IBAT +IOUT )⎤⎦

( ) ⎛

−VD • ⎝⎜ 1−

VHVOUT

VHVIN

⎞

⎠⎟

•

IBAT +IOUT )+ 0.3V •IBAT

where η is the efficiency of the high voltage regulator and

VD is the forward voltage of the catch diode at I = IBAT

+ IOUT. The first term corresponds to the power lost in

converting VHVIN to VHVOUT, 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:

PD = (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, VHVIN = 12V, VBAT

= 3.7V (VHVOUT is about 4V), IBAT = 1A, IOUT = 600mA

resulting in about 0.7W total dissipation. If the LTC4090-5

is being powered from HVIN, the power dissipation can

be estimated by calculating the regulator power loss from

an efficiency measurement, and subtracting the catch

diode loss.

4090fa

24

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