ltc4101 Linear Technology Corporation, ltc4101 Datasheet - Page 25

• Manufacturer Part Number: ltc4101
• Description: Smart Battery Charger Controller
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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 20µF) from Tokin, United Chemi-Con/Marcon, et al.
Other alternative capacitors include OSCON capacitors
from Sanyo.
U
U
W
Figure 10. Recommended SMBus Transient Protection
CONNECTOR
TO BATTERY
U
V
DD
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 0.2Ω and the battery impedance
is raised to 4Ω with a bead or inductor, only 5% of the
current ripple will flow in the battery.
Protecting SMBus Inputs
The SMBus inputs, SCL and SDA, are exposed to uncon-
trolled transient signals whenever a battery is connected
to the system. If the battery contains a static charge, the
SMBus inputs are subjected to transients which can cause
damage after repeated exposure. Also, if the battery’s
positive terminal makes contact to the connector before
the negative terminal, the SMBus inputs can be forced
below ground with the full battery potential, causing a
potential for latch-up in any of the devices connected to the
SMBus inputs. Therefore it is good design practice to
protect the SMBus inputs as shown in Figure 10.
I
RMS
=
0 29
TO SYSTEM
. (
RMS
DCIN
4101 F13
V
BAT
= 0.26A.
( )( )
L f
= 12V, V
) –
1
⎛
⎜
⎝
1
V
V
DCIN
BAT
BAT
⎞
⎟
⎠
= 4.2V, L1 = 10µH, and
LTC4101
25
4101f