LTC4054L-4.2
12
4054l42fa
Moreover, when thermal feedback reduces the charge
current, the voltage at the PROG pin is also reduced pro-
portionally as discussed in the Operation section.
It is important to remember that LTC4054L 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
Because of the small size of the ThinSOT package, it is
very important to use a good thermal PC board layout to
maximize the available charge current. The thermal path
for the heat generated by the IC is from the die to the cop-
per lead frame, through the package leads, (especially the
ground lead) to the PC board copper. The PC board copper
is the heat sink. The footprint copper pads should be as
wide as possible and expand out to larger copper areas to
spread and dissipate the heat to the surrounding ambient.
Feedthrough vias to inner or backside copper layers are
also useful in improving the overall thermal performance
of the charger. Other heat sources on the board, not related
to the charger, must also be considered when designing a
PC board layout because they will affect overall temperature
rise and the maximum charge current.
The following table lists thermal resistance for several
different board sizes and copper areas. All measurements
were taken in still air on 3/32" FR-4 board with the device
mounted on topside.
Table 1. Measured Thermal Resistance (2-Layer Board*)
COPPER AREA
BOARD
AREA
THERMAL RESISTANCE
JUNCTION-TO-AMBIENTTOPSIDE BACKSIDE
2500mm
2
2500mm
2
2500mm
2
125°C/W
1000mm
2
2500mm
2
2500mm
2
125°C/W
225mm
2
2500mm
2
2500mm
2
130°C/W
100mm
2
2500mm
2
2500mm
2
135°C/W
50mm
2
2500mm
2
2500mm
2
150°C/W
*Each layer uses one ounce copper
applicaTions inForMaTion
Table 2. Measured Thermal Resistance (4-Layer Board**)
COPPER AREA
(EACH SIDE)
BOARD
AREA
THERMAL RESISTANCE
JUNCTION-TO-AMBIENT
2500mm
2***
2500mm
2
80°C/W
**Top and bottom layers use two ounce copper, inner layers use one
ounce copper.
***10,000mm
2
total copper area
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 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, refer to Application Note 88.
Charge Current Soft-Start
The LTC4054L 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.
CHRG Status Output Pin
The CHRG pin can provide an indication that the input
voltage is greater than the undervoltage lockout threshold
level. A weak pull-down current of approximately 20µA
indicates that sufficient voltage is applied to V
CC
to begin
charging. When a discharged battery is connected to the
charger, the constant current portion of the charge cycle
begins and the CHRG pin pulls to ground. The CHRG pin
can sink up to 10mA to drive an LED that indicates that a
charge cycle is in progress.
