LTC4100_09 datasheet(25/30 Pages) LINER

25 / 30 page

LTC4100

4100fb

APPLICATIONS INFORMATION

Soft-Start and Undervoltage Lockout

The LTC4100 is soft-started by the 0.12μF capacitor on

the ITH pin. On start-up, ITH pin voltage will rise quickly

to 0.5V, then ramp up at a rate set by the internal 30μA

pull-up current and the external capacitor. Battery charging

current starts ramping up when ITH voltage reaches 0.8V

and full current is achieved with ITH at 2V. With a 0.12μF

capacitor, time to reach full charge current is about 2ms and

it is assumed that input voltage to the charger will reach

full value in less than 2ms. The capacitor can be increased

up to 1μF 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

computer 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

regulators 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

We recommend the use of high capacity low ESR/ESL X5R

type ceramic capacitors. Alternative capacitors include

OSCON or POSCAP type capacitors. Aluminum electrolytic

capacitors are not recommended for poor ESR and ESL

reasons. Solid tantalum low ESR capacitors are acceptable,

but caution must be used when tantalum capacitors are

used for input or output bypass. High input surge currents

can be created when the power adapter is hot-plugged

into the charger or when a battery is connected to the

charger. Use only “surge robust” low ESR tantalums. Re-

gardless of which type of capacitor you use, after voltage

selection, the most important thing to meet is the ripple

current requirements followed by the capacitance value.

By the time you solve the ripple current requirements,

the minimum capacitance value is often met by default.

The following equation shows the minimum COUT (±20%

tolerance) capacitance values for stability when used with

the compensation shown in the typical application on the

back page.

COUT(MIN) = 200/L1

The use of aluminum electrolytic for C1, located at the

AC adapter input terminal, is helpful in reducing ringing

during the hot-plug event. Refer to Application Note 88

for more information.

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. C2 is recommended to be equal to or greater than

C4 (output capacitor) in capacitance value.

The output capacitor (C4) 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 induc-

tors may be added to increase battery impedance at the

300kHz switching frequency. Switching ripple current splits

between 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 0.2Ω and the battery impedance

is raised to 4Ω with a bead or inductor, only 5% of the

current ripple will ﬂow in the battery.

LTC4100 Datasheet(PDF) 25 Page - Linear Technology

LTC4100 Datasheet(HTML) 25 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Datasheet Download

Link URL