LP78084 datasheet(16/20 Pages) POWER

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Preliminary Datasheet

LP78084

LP78084 – 02 Ver. 1.1 Datasheet

Nov.-2007

Page 16 of 22

Where PD

=IL OAD 2 × RDS ( O N) is the power dissipated by the

regulator ; JA is the thermal resistance from the junction of

the die to the ambient temperature.

The junction temperature, TJ , is given by:

Where TA is the ambient temperature.

TJ should be below the maximum junction temperature of

150°C.

Linear Regulator Operation:

The LP78084 includes a low-noise, low-dropout, linear

regulator operates from a 2.5V to 5.5V input and is

guaranteed to deliver 300mA.

The linear regulator is stable with small 2.2µF ceramic

capacitor.

Its

performance

suits

battery

powered

applications because of its shutdown mode, low quiescent

current, and very low dropout voltage. The low dropout

voltage allows for more utilization of a battery’s available

energy by operating closer to its end-of-life voltage.

A P PL IC AT I O N S IN F O RM AT IO N

BATTERY CHARGER

Programming Charge Current

The battery charge current is programmed using a single

resistor from the ISET pin to ground. The charge current is

400 times the current out of the ISET pin. The program

resistor and the charge current are calculated using the

following equations:

The charge current out of the BAT pin can be determined at

any time by monitoring the ISET pin voltage and using the

following equation:

Stability Considerations

The LP78084 battery charger contains two control loops:

constant-voltage and constant-current. The constant-voltage

loop is stable without any compensation when a battery is

connected with low impedance leads. Excessive lead length,

however, may add enough series inductance to require a

bypass capacitor of at least 1 F from BAT to GND.

In constant-current mode, the ISET pin voltage is in the

feedback loop, not the battery voltage. Because of the

additional pole created by ISET pin capacitance, capacitance

on this pin must be kept to a minimum. With no additional

capacitance on the ISET pin, the battery charger is stable

with ISET resistor values as high as 25k. However, additional

capacitance on this node reduces the maximum allowed

program resistor. The pole frequency at the ISET pin should

be kept above 100kHz. Therefore, if the ISET pin is loaded

with a capacitance, CISET, the following equation should be

used to calculate the maximum resistance value for RIS E T :

21 0

Average, rather than instantaneous, battery current may be

of interest to the user. For example, when the switching

regulator 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 ISET pin to measure the average

battery current as shown in Figure 3. A 10k resistor has been

added between the ISET pin and the filter capacitor to

ensure stability.

Figure 3. Isolating Capacitive Load on ISET Pin and Filtering

Undervoltage Charge Current Limiting (UVCL)

USB powered systems tend to have highly variable source

impedances (due primarily to cable quality and length). A

transient load combined with such impedance can easily trip

the UVLO threshold and turn the battery charger off unless

undervoltage charge current limiting is implemented.

Consider a situation where the LP78084 is operating under

normal conditions and the input supply voltage begins to sag

(e.g. an external load drags the input supply down). If the

input voltage reaches VUVCL (approximately 300mV above

the battery voltage,

), under-voltage charge current

limiting will begin to reduce the charge current in an

attempt to maintain

between ADP and BAT. The

LP78084 will continue to operate at the reduced charge

current until the input supply voltage is increased or voltage

mode reduces the charge current further.

Operation from Current Limited Wall Adapter

By using a current limited wall adapter as the input supply,

the LP78084 can dissipate significantly less power when

programmed for a current higher than the limit of the

supply.

Consider a situation where an application requires a 200mA

charge current for a discharged 800mAh Li-Ion battery. If a

typical 5V (non-current limited) input supply is available

then the peak power dissipation inside the part can exceed

300mW.

Now consider the same scenario, but with a 5V input supply

with a 200mA current limit. To take advantage of the supply,

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