ltc4007eufd-1-trpbf Linear Technology Corporation, ltc4007eufd-1-trpbf Datasheet - Page 15

• Manufacturer Part Number: ltc4007eufd-1-trpbf
• Description: 4a, High Efficiency, Standalone Li-ion Battery Charger
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
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 capaci-
tors 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.
Highest possible voltage rating on the capacitor will mini-
mize 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 OS-CON 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:
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 the 300kHz
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
V
f = 300kHz, I
I
RMS
DCIN
=
= 19V, V
0 29
.
(
RMS
V
BAT
BAT
U
( )( )
L f
= 0.41A.
1
)
= 12.6V, L1 = 10µH, and
⎛
⎜
⎝
1
–
U
V
V
DCIN
BAT
⎞
⎟
⎠
W
U
is raised to 4Ω with a bead or inductor, only 5% of the
current ripple will flow in the battery.
Inductor Selection
Higher operating frequencies allow the use of smaller
inductor and capacitor values. A higher frequency gener-
ally results in lower efficiency because of MOSFET gate
charge losses. In addition, the effect of inductor value on
ripple current and low current operation must also be
considered. The inductor ripple current ∆I
with higher frequency and increases with higher V
Accepting larger values of ∆I
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ∆I
∆I
CA1. Remember the maximum ∆I
mum input voltage. In practice 10µH is the lowest value
recommended for use.
Lower charger currents generally call for larger inductor
values. Use Table 4 as a guide for selecting the correct
inductor value for your application.
Table 4
L
MAX AVERAGE
∆ =
CURRENT (A)
exceed 0.6(I
I
L
1
1
2
2
3
3
4
4
( )( )
f L
1
V
MAX
OUT
INPUT VOLTAGE (V)
) due to limits imposed by I
⎛
⎜
⎝
1–
≤20
≤20
≤20
≤20
>20
>20
>20
>20
L
V
V
OUT
= 0.4(I
IN
L
⎞
⎟
⎠
allows the use of low
MAX
L
occurs at the maxi-
). In no case should
MINIMUM INDUCTOR
LTC4007
VALUE (µH)
40 ±20%
56 ±20%
20 ±20%
30 ±20%
15 ±20%
20 ±20%
10 ±20%
15 ±20%
L
decreases
REV
15
IN
.
4007fa
and