LT1571 datasheet(8/16 Pages) LINER

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LT1571 Series

Figure 3. PWM Current Programming

is achieved with VC at 1.1V. With a 0.3µF capacitor, the

time to reach full charge current is about 9ms and it is

assumed that input voltage to the charger will reach full

value in less than 3ms. Capacitance can be increased up to

µF if longer input start-up times are needed.

In any switching regulator, conventional time-based soft

starting can be defeated if the input voltage rises much

slower than the time-out period. This happens because the

switching regulators in the battery charger and the com-

puter power supply are typically supplying a fixed amount

of power to the load. If the input voltage comes up slowly

compared to the soft-start time, the regulators will try to

deliver full power to the load when the input voltage is still

well below its final value. If the adapter is current limited,

it cannot deliver full power at reduced output voltages and

the possibility exists for a quasi “latch” state where the

adapter output stays in a current limited state at reduced

output voltage. For instance, if maximum charger plus

computer load power is 20W, a 24V adapter might be

current limited at 1A. If adapter voltage is less than (20W/1A

= 20V) when full power is drawn, the adapter voltage will be

pulled down by the constant 20W load until it reaches a lower

stable state where the switching regulators can no longer

supply full load. This situation can be prevented by utilizing

undervoltage lockout, set higher than the minimum adapter

voltage where full power can be achieved.

A fixed undervoltage lockout of 7V is built into the LT1571.

A higher lockout voltage can be implemented with a Zener

diode D2 (see Figure 2).

The lockout voltage will be VIN = VZ + 1V.

For example, for a 24V adapter to start charging at 22VIN,

choose VZ = 21V. When VIN is less than 22V, D1 keeps VC

low and charger off.

Charge Current Programming

The basic formula for charge current is (see Block

Diagram):

BAT

PROG

()(

) = 





 ()

2000

2 465

2000

where RPROG is the total resistance from PROG pin to

ground.

For example, 1A charge current is needed.

PROG =

()(

)

2 465

2000

493

Charge current can also be programmed by pulse width

modulating IPROG with a switch Q1 to RPROG at a frequency

higher than a few kHz (Figure 3). Charge current will be

proportional to the duty cycle of Q1 with full current at

100% duty cycle.

When a microprocessor DAC output is used to control

charge current, it must be capable of sinking current

at a compliance up to 2.5V if connected directly to the

PROG pin.

Figure 2. Undervoltage Lockout

GND

VCC

VIN

1571 F02

LT1571

1N4148

PWM

RPROG

4.64k

300

Ω

PROG

CPROG

µF

VN2222

LT1571

1571 F03

IBAT = (DC)(1A)

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