LP78084 datasheet(17/20 Pages) POWER

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Preliminary Datasheet

LP78084

LP78084 – 02 Ver. 1.1 Datasheet

Nov.-2007

Page 17 of 22

it is necessary to program the LP78084 to charge at a

current greater than 200mA. Assume that the LP78084

charger is programmed for 300mA (i.e., RI S ET = 1 .33k) to

ensure that part tolerances maintain a programmed current

higher than 200mA. Since the battery charger will demand a

charge current higher than the current limit of the input

supply, the supply voltage will collapse to the battery

voltage plus 200mA times the on-resistance of the internal

PMOSFET. The on-resistance of the battery charger power

device is approximately 1 with a 5V supply. The actual

on-resistance will be slightly higher due to the fact that the

input supply will have collapsed to less than 5V. The

power

dissipated

during

this

phase

charging

approximately 40mW. That is a ten times improvement

over the non-current limited supply power dissipation.

USB and Wall Adapter Power

Although the LP78084 allows charging from a USB port, a

wall adapter can also be used to charge Li-Ion batteries.

Figure 4 shows an example of how to combine wall adapter

and USB power inputs. A P-channel MOSFET, MP 1, is

used to prevent back conducting into the USB port when a

wall adapter is present and Schottky diode, D1, is used to

prevent USB power loss through the 1k pulldown resistor.

Typically a wall adapter can supply significantly more

current than the current-limited USB port. Therefore, an

N-channel MOSFET, MN 1, and an extra program resistor

can be used to increase the charge current when the wall

adapter is present.

Figure 4. Combining Wall Adapter and USB Power

Power Dissipation

The conditions that cause the LP78084 battery charger to

reduce charge current through thermal feedback can be

approximated by considering the total power dissipated in

the IC. For high charge currents, the LP78084 power

dissipation is approximately:

Where PD is the total power dissipated within the IC, ADP

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

is the charge current and PD_BUCK is the power dissipation

due to the regulator. PD_BUCK can be calculated as:

Where VOUTB is the regulated output of the switching

regulator, IOUTB is the regulator load and is the regulator

efficiency at that particular load.

It is not necessary to perform worst-case power dissipation

scenarios because the LP78084 will automatically reduce

the charge current to maintain the die temperature at

approximately 115°C. However, the approximate ambient

temperature at which the thermal feedback begins to rotect

the IC is:

Example: Consider the extreme case when an LP78084 is

operating from a 6V supply providing 250mA to a 3V

Li-Ion battery, the switching regulator and the LDO are off.

The ambient temperature above which the LP78084 will

begin to reduce the 250mA charge current is approximately:

(Correctly soldered to a 2500mm

double-sided 1 oz.

copper board, the LP78084 has a thermal resistance of

approximately 43°C/W.)

T =

1 15C6V3V250 m A 43 C/W

() ()

If there is more power dissipation due to the switching

regulator or the LDO, the thermal regulation will kick in at

a somewhat lower temperature than this. In the above

circumstances, the LP78084 can be used above 82.75°C, but

the charge current will be reduced from 250mA. The

approximate current at a given ambient temperature can be

calculated:

Using the previous example with an ambient temperature of

85°C, the charge current will be reduced to approximately:

Note: 1V = 1J/C = 1W/A

Furthermore, the voltage at the ISET pin will change

proportionally with the charge current as discussed in the

Programming Charge Current section.

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