LTC4080X datasheet(17/20 Pages) LINER

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LTC4080X

4080Xf

APPLICATIO S I FOR ATIO

Where VOUT is the regulated output of the switching

regulator, IOUT is the regulator load and η is the regulator

efﬁciency at that particular load.

It is not necessary to perform worst-case power dissipa-

tion scenarios because the LTC4080X will automatically

reduce the charge current to maintain the die temperature

at approximately 115°C. However, the approximate ambi-

ent temperature at which the thermal feedback begins to

protect the IC is:

TA = 115°C – PDθJA

TA = 115°C – (VCC – VBAT) • IBAT • θJA if the regulator

is off.

Example: Consider the extreme case when an LTC4080X

is operating from a 6V supply providing 250mA to a 3V

Li-Ion battery and the regulator is off. The ambient tem-

perature above which the LTC4080X will begin to reduce

the 250mA charge current is approximately:

TA = 115°C – (6V – 3V) • (250mA) • 43°C/W

TA = 115°C – 0.75W • 43°C/W = 115°C – 32.25°C

TA = 82.75°C

If there is more power dissipation due to the regulator,

the thermal regulation will begin at a somewhat lower

temperature. In the above circumstances, the LTC4080X

can be used above 82.75°C, but the charge current will be

reduced from 250mA. The approximate current at a given

ambient temperature can be calculated:

BAT

°−

−

()

115

•

Using the previous example with an ambient temperature

of 85°C, the charge current will be reduced to approxi-

mately:

C W

BAT =

°− °

−

() °

115

129

•/

32 6

.mA

Furthermore, the voltage at the PROG pin will change

proportionally with the charge current as discussed in

the Programming Charge Current section.

VCC Bypass Capacitor

Many types of capacitors can be used for input bypassing;

however, caution must be exercised when using multi-layer

ceramic capacitors. Because of the self-resonant and high

Q characteristics of some types of ceramic capacitors, high

voltage transients can be generated under some start-up con-

ditions, such as connecting the battery charger input to a live

power source. Adding a 1Ω series resistor in series with an

X5R ceramic capacitor will minimize start-up voltage transients.

For more information, refer to Application Note 88.

SWITCHING REGULATOR

Setting the Buck Converter Output Voltage

The LTC4080X regulator compares the FB pin voltage with

an internal 0.8V reference to generate an error signal at the

output of the error ampliﬁer. A voltage divider from VOUT

to ground (as shown in the Block Diagram) programs the

output voltage via FB using the formula:

OUT =+

⎡

⎣⎢

⎤

⎦⎥

.•

Keeping the current low (<5µA) in these resistors maxi-

mizes efﬁciency, but making them too low may allow stray

capacitance to cause noise problems and reduce the phase

margin of the error amp loop. To improve the frequency

response, a phase-lead capacitor (CPL) of approximately

10pF can be used. Great care should be taken to route the

FB line away from noise sources, such as the inductor or

the SW line.

Inductor Selection

The value of the inductor primarily determines the cur-

rent ripple in the inductor. The inductor ripple

current ΔIL decreases with higher inductance and

increases with higher VIN or VOUT:

∆I

OUT

=−

⎛

⎝⎜

⎞

⎠⎟

•

Accepting larger values of ΔIL allows the use of low

inductances, but results in higher output voltage ripple,

greater core losses, and lower output current capability. A

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