SD4054 datasheet(11/14 Pages) SHOUDING

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e SD4054 to reduce charge

The SD4054 can be used above 45

SD4054 will begin to reduce the charge current is

ample: An SD4054 operating from a 5V USB supply

11-11

additional capacitance on this node reduces the

maximum allowed program resistor. The pole frequency

at the PROG pin should be kept above 100kHz.

Therefore, if the PROG pin is loaded with a capacitance,

PROG, the following equation can be used to calculate

the maximum resistance value for RPROG:

Average, rather than instantaneous, charge current may

be of interest to the user. For example, if a switching

power supply operating in low current mode is

connected in parallel with the battery, the average

current being pulled out of the BAT pin is typically of

more interest than the instantaneous current pulses. In

such a case, a simple RC filter can be used on the

PROG pin to measure the average battery current as

shown in Figure 8. A 10k resistor has been added

between the PROG pin and the filter capacitor to ensure

stability.

Power Dissipation

The conditions that cause th

current through thermal feedback can be approximated by

considering the power dissipated in the IC. Nearly all of this

power

dissipation

generated

the

internal

MOSFET—this is calculated to be approximately:

PD=(VCC－VBAT) • IBAT

where PD is the power dissipated, VCC is the input

supply voltage, VBAT is the battery voltage and IBAT is the

charge current. The approximate ambient temperature

at which the thermal feedback begins to protect the IC

is:

TA=120℃-PDθJA

TA=120℃-(VCC-VBAT) • IBAT • θ

is programmed to supply 400mA full-scale current to a

discharged Li-Ion battery with a voltage of 3.75V.

Assuming θ JA is 150 ℃ /W (see Board Layout

Considerations ), the ambient temperature at which the

approximately:

TA=120℃-(5V-3.75V) • (400mA) • 150℃/W

TA=120℃-0.5W • 150℃/W=120℃-75℃

TA=45℃

℃ ambient, but the

charge current will be reduced from 400mA. The

approximate current at a given ambient temperature can be

approximated by:

Using the

previous example

with an

ambient

temperature of 60℃, the charge current will be reduced

approximately:

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

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℃.

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 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

Figure 8. Isolating Capacitive Load on PROG Pin and Filtering

SH OU D IN G

SD 4054

SD4054-datasheet V1.0

Sep.-2006

SD4054 Datasheet(PDF) 11 Page - SHOUDING Shouding Semiconductor

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