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LTC4001

4001f

APPLICATIO S I FOR ATIO

Insert the highest power dissipation figure into the follow-

ing equation to determine maximum junction temperature:

TJ = TA + (PD • 37°C/W)

The LTC4001 includes chip overtemperature protection. If

junction temperature exceeds 160

°C (typical), the chip

will stop battery charging until chip temperature drops

below 150

°C.

Using the LTC4001 in Applications Without a Battery

The LTC4001 is normally used in end products that only

operate with the battery attached (Figure 6). Under these

conditions the battery is available to supply load transient

currents. For indefinite operation with a powered wall

adapter there are only two requirements—that the aver-

age current drawn by the load is less than the high rate

charge current, and that VBAT stays above the trickle

charge threshold when the load is initially turned on and

during other load transients. When making this determi-

nation take into account battery impedance. If battery

voltage is less than the trickle charge threshold, the

system load may be turned off until VBAT is high enough

to meet these conditions.

The situation changes dramatically with the battery re-

moved (Figure 7). Since the battery is absent, VBAT begins

at zero when a powered wall adapter is first connected to

the battery charger.

With a maximum load less than the

LTC4001 trickle charge current, battery voltage will ramp

up until VBAT crosses the trickle charge threshold. When

this occurs, the LTC4001 switches over from trickle charge

to high rate (PWM) charge mode but initially delivers zero

current (because the soft-start pin is at zero). Battery

voltage drops as a result of the system load, crossing

below the trickle charge threshold. The charger re-enters

trickle charge mode and the battery voltage ramps up

again until the battery charger re-enters high rate mode.

The soft-start voltage is slightly higher this time around

(than in the previous PWM cycle). Every successive time

that the charger enters high rate (PWM) charge mode, the

soft-start pin is at a slightly higher voltage. Eventually high

rate charge mode begins with a soft-start voltage that

causes the PWM charger to provide more current than the

system load demands, and VBAT rapidly rises until the float

voltage is reached.

For battery-less operation, system load current should be

restricted to less than the worst case trickle charge current

(preferably less than 30mA) when VBAT is less than 3.15V

(through an undervoltage lockout or other means). Above

VBAT = 3.15V, system load current less than or equal to the

high rate charge current is allowed. If operation without a

battery is required, additional low-ESR output filtering

improves start-up and other load transients. Battery-less

start-up is also improved if a 10k resistor is placed in

series with the soft-start capacitor.

Figure 6. Typical Application

WALL

ADAPTER

LTC4001

BATTERY

CHARGER

SYSTEM

LOAD

Li-Ion

BATTERY

4001 F06

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