LTC4002
13
4002f
Input and Output Capacitors
Since the input capacitor is assumed to absorb all input
switching ripple current in the converter, it must have an
adequate ripple current rating. Worst-case RMS ripple cur-
rent is approximately one-half of output charge current.
Actual capacitance value is not critical. Solid tantalum
capacitors have a high ripple current rating in a relatively
small surface mount package, but caution must be used
when tantalum capacitors are used for input bypass. High
input surge currents can be created when the adapter is
hot-plugged to the charger and solid tantalum capacitors
have a known failure mechanism when subjected to very
high turn-on surge currents. Selecting the highest pos-
sible voltage rating on the capacitor will minimize prob-
lems. Consult with the manufacturer before use.
The selection of output capacitor C
OUT
is primarily deter-
mined by the ESR required to minimize ripple voltage and
load step transients. The output ripple ∆V
OUT
is approxi-
mately bounded by:
∆≤∆ +
⎛
⎝
⎜
⎞
⎠
⎟
V I ESR
fC
OUT L
OSC OUT
1
8
Since ∆I
L
increases with input voltage, the output ripple is
highest at maximum input voltage. Typically, once the ESR
requirement is satisfied, the capacitance is adequate for
filtering and has the necessary RMS current rating.
APPLICATIO S I FOR ATIO
WUUU
Switching ripple current splits between the battery and the
output capacitor depending on the ESR of the output ca-
pacitor and the battery impedance. EMI considerations
usually make it desirable to minimize ripple current in the
battery leads. Ferrite beads or an inductor may be added
to increase battery impedance at the 500kHz switching
frequency. If the ESR of the output capacitor is 0.2Ω and
the battery impedance is raised to 4Ω with a bead or induc-
tor, only 5% of the current ripple will flow in the battery.
Design Example
As a design example, take a charger with the following
specifications: V
IN
= 5V to 22V, V
BAT
= 4V nominal, I
BAT
=
1.5A, f
OSC
= 500kHz, see Figure 2.
First, calculate the SENSE resistor :
R
SENSE
= 100mV/1.5A = 68mΩ
Choose the inductor for about 65% ripple current at the
maximum V
IN
:
L
V
kHz A
V
V
H=
()()()
⎛
⎝
⎜
⎞
⎠
⎟
=µ
4
500 0 65 1 5
1
4
22
6 713
..
–.
Selecting a standard value of 6.8µH results in a maximum
ripple current of :
∆ =
()
µ
()
⎛
⎝
⎜
⎞
⎠
⎟
=I
V
kHz H
V
V
mA
L
4
500 6 8
1
4
22
962 6
.
–.
