LTC4009-2_15 Datasheet, PDF (20/28 Pages) Linear Technology – High Efficiency, Multi-Chemistry Battery Charger

LTC4009
LTC4009-1/LTC4009-2
Applications Information
Input and Output Capacitors
In addition to typical input supply bypassing (0.1µF) on
DCIN, the relatively high ESR of aluminum electrolytic
capacitors is helpful for reducing ringing when hot plug-
ging the charger to the AC adapter. Refer to LTC Application
Note 88 for more information.
The input capacitor between system power (drain of top
FET, Figure 1) and GND is required to absorb all input PWM
ripple current, therefore it must have adequate ripple current
rating. Maximum RMS ripple current is typically one-half
of the average battery charge current. Actual capacitance
value is not critical, but using the highest possible voltage
rating on PWM input capacitors will minimize problems.
Consult with the manufacturer before use.
The output capacitor shown across the battery and ground
must also absorb PWM output ripple current. The general
formula for this capacitor current is:
I_RMS
=
0.29
•
V_BAT • ^{}1–
L1• f_PWM
V_BAT^
V_{CLP}
For example, I_RMS = 0.22A with:
V_BAT = 12.6V
V_CLP = 19V
L1 = 10µH
f_PWM = 550kHz
High capacity ceramic capacitors (20µF or more) available
from a variety of manufacturers can be used for input/out-
put capacitors. Other alternatives include OS-CON and
POSCAP capacitors from Sanyo.
Low ESR solid tantalum capacitors have high ripple cur-
rent rating in a relatively small surface mount package,
but exercise caution when using tantalum for input or
output bulk capacitors. High input surge current can be
created when the adapter is hot-plugged to the charger
or when a battery is connected to the charger. Solid tan-
talum capacitors have a known failure mechanism when
subjected to very high surge currents. Select tantalum
capacitors that have high surge current ratings or have
been surge tested.
EMI considerations usually make it desirable to minimize
ripple current in battery leads. Adding Ferrite beads or
inductors can increase battery impedance at the nominal
550KHz switching frequency. Switching ripple current splits
between the battery and the output capacitor in inverse
relation to capacitor ESR and the battery impedance. If
the ESR of the output capacitor is 0.2Ω and the battery
impedance is raised to 4Ω with a ferrite bead, only 5% of
the current ripple will flow to the battery.
Inductor Selection
Higher switching frequency generally results in lower
efficiency because of MOSFET gate charge losses, but it
allows smaller inductor and capacitor values to be used.
A primary effect of the inductor value L1 is the amplitude
of ripple current created. The inductor ripple current ΔI_L
decreases with higher inductance and PWM operating
frequency:
∆I_L
=
V_BAT
•
_1–
V_BAT ^
V_{CLP }
L1• f_PWM
Accepting larger values of ΔI_L allows the use of low in-
ductance, but results in higher output voltage ripple and
greater core losses. Lower charge currents generally call
for larger inductor values.
4009fd
20