LTC4100 Linear Technology, LTC4100 Datasheet - Page 20

• Manufacturer Part Number: LTC4100
• Description: Smart Battery Charger Controller
• Manufacturer: Linear Technology

LTC4100
APPLICATIO S I FOR ATIO
20
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
U
U
W
U
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 F) 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:
For example, V
f = 300kHz, I
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 and the battery impedance
is raised to 4 with a bead or inductor, only 5% of the
current ripple will flow in the battery.
I
RMS
0 29
. (
RMS
DCIN
V
BAT
= 0.41A.
( )( )
= 19V, V
L f
) –
1
1
V
V
DCIN
BAT
BAT
= 12.6V, L1 = 10 H, and
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