FC5754 datasheet(16/19 Pages) FS

16 / 19 page

FC5754

2014. 11. 04

16/19

Revision No : 0

approximate current at a given ambient temperature

can be approximated by:

BAT

VIN

∗

−

)

(

120

Using the previous example with an ambient

temperature of 70oC, the charge current will be

reduced to approximately:

BAT

384

130

100

)

(

120

∗

−

Moreover, when thermal feedback reduces the charge

current, the voltage at the PROG pin is also reduced

proportionally as discussed in the Operation section. It is

important to remember that FC5754 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 120oC.

Thermal Considerations

Because of the small size of the SOT 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 copper 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.

Increasing Thermal Regulation Current

Reducing the voltage drop across the internal MOSFET

can significantly decrease the power dissipation in the

IC. This has the effect of increasing the current delivered

to the battery during thermal regulation. One method is

by dissipating some of the power through an external

component, such as a resistor or diode.

Example: An FC5754 operating from a 5V wall

adapter is programmed to supply 1A full-scale current

to a discharged Li-Ion battery with a voltage of 3.7V.

Assuming θJA is 100oC/W, the approximate charge

current at an ambient temperature of 25oC is:

BAT

730

100

)

(

120

∗

−

By dropping voltage across a resistor in series with a 5V

wall adapter (shown in Figure 2), the on-chip power

dissipation can be decreased, thus increasing the

thermally regulated charge current

BAT

∗

−

)

(

120

Solving for IBAT using the quadratic formula.

]

)

120

(

)

(

)

[(

BAT

−

(Note: Large values of RCC will result in no solution for IBAT This indicates that the

FC5754 will not generate enough heat to require thermal regulation )

Using RCC = 0.25Ω, VS = 5V, VBAT = 3.7V, TA = 25°C and JA

= 100°C/W we can calculate the thermally regulated

charge current to be:

IBAT = 879.5mA

While this application delivers more energy to the

battery and reduces charge time in thermal mode, it

may actually lengthen charge time in voltage mode if

VIN becomes low enough to put the FC5754 into

dropout.

This technique works best when RCC values are

minimized to keep component size small and avoid

dropout. Remember to choose a resistor with adequate

power handling capability.

VIN 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.

Charge Current Soft-Start

The FC5754 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 100us. This has the effect of minimizing

the transient current load on the power supply during

start-up.

Figure 2. A Circuit to Maximize Thermal Mode Charge Current

FC5754 Datasheet(PDF) 16 Page - First Silicon Co., Ltd

FC5754 Datasheet(HTML) 16 Page - First Silicon Co., Ltd

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Datasheet Download

Link URL