LTC4100 datasheet(20/24 Pages) LINER

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LTC4100

sn4100 4100is

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 30W, a 15V adapter might be

current limited at 2.5A. If adapter voltage is less than

(30W/2.5A = 12V) when full power is drawn, the adapter

voltage will be pulled down by the constant 30W load until

it reaches a lower stable state where the switching regu-

lators can no longer supply full load. This situation can be

prevented by utilizing the DCDIV resistor divider, set

higher than the minimum adapter voltage where full power

can be achieved.

Input and Output Capacitors

In the 4A Lithium Battery Charger (Typical Application on

back page), the input capacitor (C2) is assumed to absorb

all input switching ripple current in the converter, so it

must have adequate ripple current rating. Worst-case

RMS ripple current will be equal to one half of output

charging current. Actual capacitance value is not critical.

Solid tantalum low ESR capacitors have high ripple cur-

rent rating in a relatively small surface mount package,

but

caution must be used when tantalum capacitors are used

for input or output bypass. High input surge currents can

be created when the adapter is hot-plugged to the charger

or when a battery is connected to the charger. Solid

tantalum capacitors have a known failure mechanism

when subjected to very high turn-on surge currents. Only

Kemet T495 series of “Surge Robust” low ESR tantalums

are rated for high surge conditions such as battery to

ground.

The relatively high ESR of an aluminum electrolytic for C1,

located at the AC adapter input terminal, is helpful in

reducing ringing during the hot-plug event. Refer to AN88

for more information.

The highest possible voltage rating on the capacitor will

minimize problems. Consult with the manufacturer before

use. Alternatives include new high capacity ceramic (at

least 20

µF) from Tokin, United Chemi-Con/Marcon, et al.

Other alternative capacitors include OSCON capacitors

from Sanyo.

The output capacitor (C3) is also assumed to absorb

output switching current ripple. The general formula for

capacitor current is:

RMS

BAT

DCIN









029

) –

()( )

For example, VDCIN = 19V, VBAT = 12.6V, L1 = 10µH, and

f = 300kHz, IRMS = 0.41A.

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 the300kHz

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 imped-

ance. If the ESR of C3 is 0.2

Ω and the battery impedance

is raised to 4

Ω with a bead or inductor, only 5% of the

current ripple will flow in the battery.

APPLICATIO S I FOR ATIO

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