LTC4100
25
4100fc
For more information www.linear.com/LTC4100
applicaTions inForMaTion
Soft-Start and Undervoltage Lockout
The LTC4100 is soft-started by the 0.12µF capacitor on
the I
TH
pin. On start-up, I
TH
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 I
TH
voltage reaches 0.8V
and full current is achieved with I
TH
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 con-
nected to
the charger. Use only “surge robust” low ESR
tantalums.
Regardless 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 C
OUT
(±20% tolerance) capacitance values for
stability when used with the compensation shown in the
typical application on the back page.
C
OUT(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
=
0.29(V
BAT
) • 1−
V
BAT
V
DCIN
L1• f
For example, V
DCIN
= 19V, V
BAT
= 12.6V, L1 = 10µH, and
f = 300kHz, I
RMS
= 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.
