9

LT1510/LT1510-5

capacity ceramic capacitor (5

µF to 10µF) from Tokin or

United Chemi-Con/MARCON, et al., and the old standby,

aluminum electrolytic, which will require more microfarads

to achieve adequate ripple rating. OS-CON can also be used.

output switching current ripple. The general formula for

capacitor current is:

I

V

V

V

Lf

RMS

BAT

BAT

CC

=









()( )

029

1

1

.

EMI considerations usually make it desirable to minimize

ripple current in the battery leads, and beads or inductors

may be added to increase battery impedance at the 200kHz

switching frequency. Switching ripple current splits be-

tween the battery and the output capacitor depending on

the ESR of the output capacitor and the battery impedance.

raised to 4

Ωwithabeadofinductor,only5%ofthecurrent

ripple will flow in the battery.

Soft Start

The LT1510 is soft started by the 0.1

then ramp at a rate set by the internal 45

µA pull-up current

and the external capacitor. Battery charging current starts

reach full charge current is about 3ms and it is assumed

that input voltage to the charger will reach full value in less

than 3ms. Capacitance can be increased up to 0.47

µF if

longer input start-up times are needed.

In any switching regulator, conventional timer-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 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

sucked 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

undevoltage lockout, set higher than the minimum

adapter voltage where full power can be achieved.

pin rises above 7V. The charger will start delivering current

pin. Higher lockout voltage can be implemented with a

Zener diode (see Figure 3 circuit).

Figure 3. Undervoltage Lockout

GND

1510 F03

LT1510

2k

D1

1N4001

low and charger off.

Charging Current Programming

The basic formula for charging current is (see Block

Diagram):

II

V

R

BAT

PROG

PROG

=







2000

2 465

2000

.