LTC4075HVXEDD 查看數據表（PDF） - Linear Technology

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LTC4075HVXEDD

High Voltage Dual Input Li-Ion/Polymer Battery Charger

Linear Technology 

LTC4075HVX

APPLICATIONS INFORMATION

Assuming θJA is 40°C/W (see Thermal Considerations),

the ambient temperature at which the LTC4075HVX will

begin to reduce the charge current is approximately:

TA = 125°C – (5V – 3.3V) • (800mA) • 40°C/W

TA = 125°C – 1.36W • 40°C/W = 125°C – 54.4°C

TA = 70.6°C

The LTC4075HVX can be used above 70.6°C ambient,

but the charge current will be reduced from 800mA. The

approximate current at a given ambient temperature can

be approximated by:

IBAT

=

125°C – TA

(VIN – VBAT) • θJA

Using the previous example with an ambient temperature

of 80°C, the charge current will be reduced to approxi-

mately:

IBAT

=

(5V

125°C – 80°C

– 3.3V) • 40°C

/

W

=

45°C

68°C / A

IBAT = 662mA

It is important to remember that LTC4075HVX applica-

tions do not need to be designed for worst-case thermal

conditions, since the IC will automatically reduce power

dissipation when the junction temperature reaches ap-

proximately 125°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 LTC4075HVX DFN package is properly sol-

dered to the PC board ground. When correctly soldered to a

2500mm2 double sided 1oz copper board, the LTC4075HVX

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 ex-

ample, a correctly soldered LTC4075HVX 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 prod-

uct, the inductance of the cable and the high-Q ceramic

input capacitor form an L-C resonant circuit. While the

LTC4075HVX is 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 twice the input voltage before it settles

out. To prevent excessive voltage from damaging the

LTC4075HVX during a hot insertion, it is best to have a

low voltage coefficient capacitor at the input pins to the

LTC4075HVX. This is achievable by selecting 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.

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 recommended.

Alternatively, the following soft connect circuit can be

employed (as shown in Figure 4).

R1

15V

39k

INPUT INPUT CABLE

C1

C2

1μF

100nF

MN1

DCIN/USBIN

LTC4075HVX

GND

4075hvx F04

Figure 4. Input Soft Connect Circuit

4075hvxf

13

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