Datasheet

LTC4007
15
4007fc
APPLICATIONS INFORMATION
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. 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
minimize problems. Consult with the manufacturer before
use. Alternatives include 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:
I
V
V
V
Lf
RMS
BAT
BAT
DCIN
=
()
⎛
⎝
⎜
⎞
⎠
⎟
()()
029 1
1
.–
For example:
V
DCIN
= 19V, V
BAT
= 12.6V, L1 = 10μH, and
f = 300kHz, I
RMS
= 0.41A.
EMI considerations usually make it desirable to minimize
ripple current in the battery leads, and beads or induc-
tors may be added to increase battery impedance at the
300kHz switching frequency. Switching ripple current splits
between 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 fl ow 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 effi ciency 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 ΔI
L
decreases with
higher frequency and increases with higher V
IN
.
Δ=
()()
⎛
⎝
⎜
⎞
⎠
⎟
I
fL
V
V
V
L OUT
OUT
IN
1
1–
Accepting larger values of ΔI
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is ΔI
L
= 0.4(I
MAX
). In no case should
ΔI
L
exceed 0.6(I
MAX
) due to limits imposed by I
REV
and
CA1. Remember the maximum ΔI
L
occurs at the maxi-
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
MAX AVERAGE
CURRENT (A) INPUT VOLTAGE (V)
MINIMUM INDUCTOR
VALUE (μH)
1 ≤20 40 ±20%
1 >20 56 ±20%
2 ≤20 20 ±20%
2 >20 30 ±20%
3 ≤20 15 ±20%
3 >20 20 ±20%
4 ≤20 10 ±20%
4 >20 15 ±20%