LTC4008_15 Datasheet, PDF (15/24 Pages) Linear Technology – 4A, High Effi ciency, Multi-Chemistry Battery Charger

LTC4008
APPLICATIONS INFORMATION
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
Application Note 88 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_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 between
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 ΔI_L decreases with
higher frequency and increases with higher V_IN.
ΔI_L
=
1
(^f)(^L)
V_OUT
⎛⎝⎜_1–
V_OUT
V_IN
⎞
⎠⎟
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
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.
4008fb
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