LTC4008 Datasheet, PDF (14/24 Pages) Linear Technology – 4A, High Efficiency, Multi-Chemistry Battery Charger

LTC4008
APPLICATIO S I FOR ATIO
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 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 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.
Table 4
MAXIMUM
AVERAGE CURRENT (A)
1
1
2
2
3
3
4
4
INPUT
VOLTAGE (V)
≤ 20
> 20
≤ 20
> 20
≤ 20
> 20
≤ 20
> 20
MINIMUM INDUCTOR
VALUE (µH)
40 ±20%
56 ±20%
20 ±20%
30 ±20%
15 ±20%
20 ±20%
10 ±20%
15 ±20%
Charger Switching Power MOSFET
and Diode Selection
Two external power MOSFETs must be selected for use
with the charger: a P-channel MOSFET for the top (main)
switch and an N-channel MOSFET for the bottom (syn-
chronous) switch.
The peak-to-peak gate drive levels are set internally. This
voltage is typically 6V. Consequently, logic-level threshold
MOSFETs must be used. Pay close attention to the BV_DSS
specification for the MOSFETs as well; many of the logic
level MOSFETs are limited to 30V or less.
4008fa
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