LTC4058-4.2_15 Datasheet, PDF (10/12 Pages) Linear Technology – Standalone Linear Li-Ion Battery Charger with Thermal Regulation in DFN

LTC4058-4.2/LTC4058X-4.2
APPLICATIO S I FOR ATIO
The LTC4058 can be used above 52°C ambient but the
charge current will be reduced from 800mA. The approxi-
mate current at a given ambient temperature can be
approximated by:
( ) ^{IBAT =}
120°C – T_A
V_CC – V_BAT • θ_JA
Using the previous example with an ambient temperature
of 60°C, the charge current will be reduced to
approximately:
I_BAT
=
(^5V
120°C –
– 3.3V₎
60°C
• 50°C/W
=
60°C
85°C/A
I_BAT = 706mA
Moreover, when thermal feedback reduces the charge cur-
rent the voltage at the PROG pin is also reduced proportion-
ally as discussed in the Operation section. It is important
to remember that LTC4058 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 LTC4058 package is soldered to the PC
board ground. Correctly soldered to a 2500mm² double-
sided 1oz copper board, the LTC4058 has a thermal
resistance of approximately 40°C/W. Failure to make
thermal contact between the exposed pad on the back-
side of the package and the copper board will result in
thermal resistances far greater than 40°C/W. As an
example, a correctly soldered LTC4058 can deliver over
800mA to a battery from a 5V supply at room tempera-
ture. Without a backside thermal connection, this num-
ber will drop considerably.
V_CC Bypass Capacitor
Many types of capacitors can be used for input bypassing,
however, caution must be exercised when using multilayer
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 charger input
to a live power source. Adding a 1.5Ω resistor in series
with an X5R ceramic capacitor will minimize start-up
voltage transients. For more information, see Application
Note 88.
Charge Current Soft-Start
The LTC4058 includes a soft-start circuit to minimize the
inrush current at the start of a charge cycle. When a charge
cycle is initiated, the charge current ramps from zero to
the full-scale current over a period of approximately 100µs.
This has the effect of minimizing the transient current load
on the power supply during start-up.
USB and Wall Adapter Power
The LTC4058 allows charging from both a wall adapter
and a USB port. Figure 3 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 a
Schottky diode, D1, is used to prevent USB power loss
through the 1k pull-down resistor.
Typically a wall adapter can supply more current than the
500mA-limited USB port. Therefore, an N-channel
MOSFET, MN1, and an extra 3.3k program resistor are
used to increase the charge current to 800mA when the
wall adapter is present.
5V WALL
ADAPTER
800mA I_CHG
USB POWER
500mA I_{CHG MP1}
1k
D1
LTC4058-4.2 ₂
BAT
6
1
4, 9 ^{VCC BSENSE} 5
GND PROG
3.3k
MN1
2k
I_CHG
SYSTEM
LOAD
+ _Li-Ion
BATTERY
405842 F03
Figure 3. Combining Wall Adapter and USB Power
10
sn405842 405842fs