LTC4012-2 Linear Technology Corporation, LTC4012-2 Datasheet - Page 21

• Manufacturer Part Number: LTC4012-2
• Description: Multi-Chemistry Battery Charger
• Manufacturer: Linear Technology Corporation

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APPLICATIONS INFORMATION
The output capacitor shown across the battery and ground
must also absorb PWM output ripple current. The general
formula for this capacitor current is:
For example, I
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 fl ow to the battery.
V
V
L1 = 10μH
f
I
PWM
RMS
BAT
CLP
= 12.6V
= 19V
= 550kHz
=
0 29
.
RMS
•
V
L
BAT
= 0.22A with:
1
•
f
•
PWM
⎛
⎝ ⎜
1
–
V
V
CLP
BAT
⎞
⎠ ⎟
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
higher inductance and PWM operating frequency:
Accepting larger values of ΔI
ductance, but results in higher output voltage ripple and
greater core losses. Lower charge currents generally call
for larger inductor values.
The LTC4012 limits maximum instantaneous peak inductor
current during every PWM cycle. To avoid unstable switch
waveforms, the ripple current must satisfy:
so choose:
A reasonable starting point for setting ripple current
is ΔI
LTC4012 circuits also imposes limits on ripple current.
Select R
high ripple designs. The following equation can be used
for guidance:
L
ΔI
ΔI
R
1
L
SENSE
L
L
>
= 0.4 • I
=
<
50
f
IN
PWM
2
V
μA
BAT
(in Figure 1) to avoid average current errors in
•
•
LTC4012-1/LTC4012-2
⎛
⎜
⎝
Δ
L
R
150
0 125
•
I
1
• –
SENSE
L
.
⎛
⎝ ⎜
MAX
•
⎛
⎜
⎝
R
150
1
≤
mV
f
PWM
SENSE
R
. The voltage compliance of internal
•
mV
IN
V
V
–
V
BAT
CLP
CLP
≤
I
MAX
–
R
⎞
⎟
⎠
I
SENSE
MAX
⎞
⎟
⎠
20
L
⎞
⎠ ⎟
allows the use of low in-
μA
•
Δ
I
L
L
LTC4012/
decreases with
21
4012f