LTC4006_15 Datasheet, PDF (13/20 Pages) Linear Technology – 4A, High Efficiency, Standalone Li-Ion Battery Charger

LTC4006
APPLICATIO S I FOR ATIO
Soft-Start
The LTC4006 is soft started by the 0.12µF capacitor on the
I_TH pin. On start-up, I_TH pin voltage will rise quickly to 0.5V,
then ramp up at a rate set by the internal 40µA pull-up
current and the external capacitor. Battery charging
current starts ramping up when I_TH voltage reaches 0.8V
and full current is achieved with I_TH at 2V. With a 0.12µF
capacitor, time to reach full charge current is about 2ms
and it is assumed that input voltage to the charger will
reach full value in less than 2ms. The capacitor can be
increased up to 1µF if longer input start-up times are
needed.
Input and Output Capacitors
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 current rating in a rela-
tively 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 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 Appli-
cation Note 88 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:
I_RMS
=
0.29₍V_{BAT )}^{⎛⎝⎜}1–
(^L1)(^f)
V_BAT
V_DCIN
⎞
⎠⎟
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 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 impedance.
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.
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_L decreases
with higher frequency and increases with higher V_IN.
∆I_L
=
1
(^f)(^L)
V_OUT
⎛⎝⎜_1–
V_OUT
V_IN
⎞
⎠⎟
Accepting larger values of ∆I_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 ∆I_L = 0.4(I_MAX). In no case should
∆I_L exceed 0.6(I_MAX) due to limits imposed by I_REV and
CA1. Remember the maximum ∆I_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.
4006fa
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