LP4062 datasheet(10/13 Pages) POWER

10 / 13 page

Preliminary

Datasheet

LP4062

LP4062 –00

Ver. 1.0 Datasheet

Nov.-2007

Page 10 of 13

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 the LP4062 to reduce charge

current through thermal feedback can be approximated

by considering the power dissipated in the IC. Nearly

all of this power dissipation is generated by 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 • θJA

Example: An LP4062 operating from a 5V USB

supply 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 LP4062 will begin to

reduce the charge current is approximately:

TA=120℃-(5V-3.75V) • (400mA) • 150℃/W

TA=120℃-0.5W • 150℃/W=120℃-75℃

TA=45℃

The LP4062 can be used above 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

to 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 that LP4062 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℃.

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 One method is by dissipating

some of the power through an external component,

such as a resistor or diode. Example: An LP4062

operating from a 5V wall adapter is programmed

to supply 800mA full-scale current to a discharged

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

θJA is 125℃/W, the approximate charge current

ambient

temperature

25 ℃

is:

By dropping voltage across a resistor in series with a

5V wall adapter (shown in Figure 9), the on-chip

power dissipation can be decreased, thus increasing the

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