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LTC4080EDD#PBF

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LTC4080
16
4080fc
For more information www.linear.com/LTC4080
APPLICATIONS INFORMATION
V
CC
MP1
MN1
1k
2k
1.33k
1
I
CHG
2
D1
4
Li-Ion
BATTERY
SYSTEM
LOAD
4080 F02
LTC4080
BAT
USB
POWER
(200mA)
5V WALL
ADAPTER
(300mA)
PROG
+
Figure 2. Combining Wall Adapter and USB Power
Undervoltage Charge Current Limiting (UVCL)
USB powered systems tend to have highly variable source
impedances (due primarily to cable quality and length). A
transient load combined with such impedance can easily
trip the UVLO threshold and turn the battery charger off un-
less undervoltage charge current limiting is implemented.
Consider a situation where the LTC4080 is operating under
normal conditions and the input supply voltage begins to
sag (e.g. an external load drags the input supply down).
If the input voltage reaches V
UVCL
(approximately 300mV
above the battery voltage, DV
UVCL
), undervoltage charge
current limiting will begin to reduce the charge current in
an attempt to maintain DV
UVCL
between V
CC
and BAT. The
LTC4080 will continue to operate at the reduced charge
current until the input supply voltage is increased or volt-
age mode reduces the charge current further.
Operation from Current Limited Wall Adapter
By using a current limited wall adapter as the input sup-
ply, the LTC4080 can dissipate significantly less power
when programmed for a current higher than the limit of
the supply.
Consider a situation where an application requires a 200mA
charge current for a discharged 800mAh Li-Ion battery.
If a typical 5V (non-current limited) input supply is avail-
able then the peak power dissipation inside the part can
exceed 300mW.
Now consider the same scenario, but with a 5V input sup-
ply with a 200mA current limit. To take advantage of the
supply, it is necessary to program the LTC4080 to charge
at a current greater than 200mA. Assume that the LTC4080
charger is programmed for 300mA (i.e., R
PROG
= 1.33kW)
to ensure that part tolerances maintain a programmed
current higher than 200mA. Since the battery charger will
demand a charge current higher than the current limit of
the input supply, the supply voltage will collapse to the
battery voltage plus 200mA times the on-resistance of the
internal PFET. The on-resistance of the battery charger
power device is approximately 0.75W with a 5V supply.
The actual on-resistance will be slightly higher due to the
fact that the input supply will have collapsed to less than
5V. The power dissipated during this phase of charging
is approximately 30mW. That is a ten times improvement
over the non-current limited supply power dissipation.
USB and Wall Adapter Power
Although the LTC4080 allows charging from a USB port,
a wall adapter can also be used to charge Li-Ion batter-
ies. Figure 2 shows an example of how to combine wall
adapter and USB power inputs. A P-channel MOSFET,
MP1, is used to prevent back conducting into the USB
port when a wall adapter is present and Schottky diode,
D1, is used to prevent USB power loss through the 1k
pull-down resistor.
Typically a wall adapter can supply significantly more
current than the current-limited USB port. Therefore, an
N-channel MOSFET, MN1, and an extra program resistor
can be used to increase the charge current when the wall
adapter is present.
Power Dissipation
The conditions that cause the LTC4080 battery charger to
reduce charge current through thermal feedback can be
approximated by considering the total power dissipated
in the IC. For high charge currents, the LTC4080 power
dissipation is approximately:
P
D
= V
CC
− V
BAT
( )
•I
BAT
+P
D _BUCK
Where P
D
is the total power dissipated within the IC, V
CC
is the input supply voltage, V
BAT
is the battery voltage, I
BAT
is the charge current and P
D_BUCK
is the power dissipation
due to the regulator. P
D_BUCK
can be calculated as:
P
D
_BUCK
=V
OUT
•I
OUT
1
h
−1
LTC4080
17
4080fc
For more information www.linear.com/LTC4080
APPLICATIONS INFORMATION
Where V
OUT
is the regulated output of the switching
regulator, I
OUT
is the regulator load and
h
is the regulator
efficiency at that particular load.
It is not necessary to perform worst-case power dissipa-
tion scenarios because the LTC4080 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:
T
A
= 115°C – P
D
θ
JA
T
A
= 115°C – (V
CC
– V
BAT
) • I
BAT
• θ
JA
if the regulator
is off.
Example: Consider the extreme case when an LTC4080 is
operating from a 6V supply providing 250mA to a 3V Li-Ion
battery and the regulator is off. The ambient temperature
above which the LTC4080 will begin to reduce the 250mA
charge current is approximately:
T
A
= 115°C – (6V – 3V) • (250mA) • 43°C/W
T
A
= 115°C – 0.75W • 43°C/W = 115°C – 32.25°C
T
A
= 82.75°C
If there is more power dissipation due to the regulator,
the thermal regulation will kick in at a somewhat lower
temperature than this. In the above circumstances, the
LTC4080 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:
I
BAT
=
115°C− T
A
V
CC
− V
BAT
( )
•θ
JA
Using the previous example with an ambient temperature of
85°C, the charge current will be reduced to approximately:
I
BAT
=
115°C−85°C
6V−3V
( )
• 43°C/W
=
30°C
129°C/A
=232.6mA
Note: 1V = 1J/C = 1W/A
Furthermore, the voltage at the PROG pin will change
proportionally with the charge current as discussed in
the Programming Charge Current section.
V
CC
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
conditions, such as connecting the battery charger input to
a live power source. Adding a 1
W
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 LTC4080 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
V
OUT
to ground (as shown in the Block Diagram) programs
the output voltage via FB using the formula:
V
OUT
=0.8V • 1+
R7
R8
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 (C
PL
) 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 cur-
rent DI
L
decreases with higher inductance and
increases with higher V
IN
or V
OUT
:
DI
L
=
V
OUT
f
0
•L
• 1−
V
OUT
V
IN
Accepting larger values of DI
L
allows the use of low
inductances, but results in higher output voltage ripple,
LTC4080
18
4080fc
For more information www.linear.com/LTC4080
APPLICATIONS INFORMATION
greater core losses, and lower output current capability.
A reasonable starting point for setting ripple current is DI
L
=0.3 • I
LIM
, where I
LIM
is the peak switch current limit.
The largest ripple current occurs at the maximum input
voltage. To guarantee that the ripple current stays below a
specified maximum, the inductor value should be chosen
according to the following equation:
L ≥
V
OUT
f
0
•DI
L
• 1−
V
OUT
V
IN
MAX
( )
For applications with V
OUT
= 1.8V, the above equation
suggests that an inductor of at least 6.8µH should be used
for proper operation.
Many different sizes and shapes of inductors are
available from numerous manufacturers. To maximize
efficiency, choose an inductor with a low DC resistance.
Keep in mind that most inductors that are very thin or
have a very small volume typically have much higher
core and DCR losses and will not give the best efficiency.
Also choose an inductor with a DC current rating at least
1.5 times larger than the peak inductor current limit to
ensure that the inductor does not saturate during nor-
mal operation. To minimize radiated noise, use a toroid,
or shielded pot core inductors in ferrite or permalloy
materials. Table 1 shows a list of several inductor manu-
facturers.
Table 1. Recommended Surface Mount Inductor Manufacturers
Coilcraft www.coilcraft.com
Sumida www.sumida.com
Murata www.murata.com
Toko www.tokoam.com
Input and Output Capacitor Selection
Since the input current waveform to a buck converter is a
square wave, it contains very high frequency components.
It is strongly recommended that a low equivalent series
resistance (ESR) multilayer ceramic capacitor be used to
bypass the BAT pin which is the input for the converter.
Tantalum and aluminum capacitors are not recommended
because of their high ESR. The value of the capacitor on
BAT directly controls the amount of input voltage ripple for
a given load current. Increasing the size of this capacitor
will reduce the input ripple.
To prevent large V
OUT
voltage steps during transient
load conditions, it is also recommended that a ceramic
capacitor be used to bypass V
OUT
. The typical value for
this capacitor is 4.7µF.
Multilayer Ceramic Chip Capacitors (MLCC) typically have
exceptional ESR performance. MLCCs combined with a
carefully laid out board with an unbroken ground plane
will yield very good performance and low EMI emissions.
There are several types of ceramic capacitors with con-
siderably different characteristics. Y5V and X5R ceramic
capacitors have apparently higher packing density but
poor performance over their rated voltage or temperature
ranges. Under given voltage and temperature conditions,
Y5V, X5R and X7R ceramic capacitors should be com-
pared directly by case size rather than specified value for
a desired minimum capacitance. Some manufacturers
provide excellent data on their websites about achiev-
able capacitance. Table 2 shows a list of several ceramic
capacitor manufacturers.
Table 2. Recommended Ceramic Capacitor Manufacturers
Taiyo Yuden www.t-yuden.com
AVX www.avxcorp.com
Murata www.murata.com
TDK www.tdk.com
Board Layout Considerations
To be able to deliver maximum charge current under all
conditions, it is critical that the exposed metal pad on the
backside of the LTC4080’s package has a good thermal
contact to the PC board ground. Correctly soldered to a
2500mm
2
double-sided 1 oz. copper board, the LTC4080
has a thermal resistance of approximately 43°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 43°C/W.
Furthermore due to its high frequency switching circuitry,
it is imperative that the input capacitor, BAT pin capaci-
tor, inductor, and the output capacitor be as close to the
LTC4080 as possible and that there is an unbroken ground
plane under the LTC4080 and all of its high frequency
components.
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LTC4080EDD#PBF

Mfr. #:
Buy LTC4080EDD#PBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Battery Management 500mA Standalone Li-Ion Charger w/ Integrated 300mA Synchronous Buck n 3x3 DFN
Lifecycle:
New from this manufacturer.
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