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LTC4078/LTC4078X

4078xfb

APPLICATIONS INFORMATION

Assuming

θJAis40°C/W(seeThermalConsiderations),the

ambient temperature at which the LTC4078/LTC4078X will

begin to reduce the charge current is approximately:

TA = 120°C – (5V – 3.3V) • (800mA) • 40°C/W

TA = 120°C – 1.36W • 40°C/W = 120°C – 54.4°C

TA = 65.6°C

The LTC4078/LTC4078X can be used above 70.6°C ambi-

ent, but the charge current will be reduced from 800mA.

The approximate current at a given ambient temperature

can be approximated by:

IBAT =

120

°C– T

(VIN –VBAT) • JA

Using the previous example with an ambient temperature

of 75°C, the charge current will be reduced to approxi-

mately:

IBAT =

120

°C– 75°C

(5V – 3.3V) • 40

°C/ W

°C

°C/ A

IBAT = 662mA

It is important to remember that LTC4078/LTC4078X

applications do not need to be designed for worst-case

thermal conditions, since the IC will automatically reduce

power dissipation when the junction temperature reaches

approximately 120°C.

Thermal Considerations

In order to deliver maximum charge current under all

conditions, it is critical that the exposed metal pad on

the backside of the LTC4078/LTC4078X DFN package is

properly soldered to the PC board ground. When cor-

rectly soldered to a 2500mm2 double sided 1oz copper

board, the LTC4078/LTC4078X has a thermal resistance

of approximately 40°C/W. Failure to make thermal contact

between the exposed pad on the backside of the package

and the copper board will result in thermal resistances far

greater than 40°C/W. As an example, a correctly soldered

Figure 4. Input Soft Connect Circuit

LTC4078/LTC4078X can deliver over 800mA to a battery

from a 5V supply at room temperature. Without a good

backside thermal connection, this number would drop to

much less than 500mA.

Input Capacitor Selection

When an input supply is connected to a portable product,

the inductance of the cable and the high-Q ceramic input

capacitor form an L-C resonant circuit. While the LTC4078/

LTC4078X are capable of withstanding input voltages as

high as 22V, if the input cable does not have adequate

mutual coupling or if there is not much impedance in

the cable, it is possible for the voltage at the input of the

product to reach as high as 2x the input voltage before it

settles out. To prevent excessive voltage from damaging

the LTC4078/LTC4078X during a hot insertion, it is best to

have a low voltage coefﬁcient capacitor at the input pins

to the LTC4078/LTC4078X. This is achievable by select-

ing an X5R or X7R ceramic capacitor that has a higher

voltage rating than that required for the application. For

example, if the maximum expected input voltage is 15V,

a 25V X5R 1μF capacitor would be a better choice than

the smaller 16V X5R capacitor. Note that no charging will

occur with 15V in.

Using a tantalum capacitor or an aluminum electrolytic

capacitor for input bypassing, or paralleling with a ceramic

capacitor will also reduce voltage overshoot during a hot

insertion. Ceramic capacitors with Y5V or Z5U dielectrics

are not recommeded.

Alternatively, the following soft connect circuit can be

employed (as shown in Figure 4).

LTC4078

DCIN/USBIN

INPUT CABLE

+15V

INPUT

1μF

100nF

MN1

4078X F04

GND

40k

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