Rainbow Electronics MAX8730 - Manual (Page 25)

MAX8730
Low-Cost Battery Charger
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where θ
JA is the thermal resistance of the package (in
°C/W), T J_MAX is the maximum junction temperature of
the diode, T A_MAX is the maximum ambient tempera-
ture of the system, and V F is the forward voltage of the
Schottky diode.
The Schottky size and cost can be reduced by utilizing
the MAX8730 foldback function. See the Trickle Charge
section for more information.
Select the Schottky diode to minimize the battery leakage
current when the charger is shut down.
Inductor Selection
The MAX8730 uses a fixed inductor current ripple
architecture to minimize the inductance. The charge
current, ripple, and operating frequency (off-time)
affects inductor selection. For a good trade-off of
inductor size and efficiency, choose the inductance
according to the following equation:
where k
OFF
is the off-time constant (5.6V x µs typically).
Higher inductance values decrease the RMS current at
the cost of inductor size.
Inductor L1 must have a saturation current rating of at
least the maximum charge current plus 1/2 of the ripple
current (
∆I
L ):
I SAT = I CHG + (1/2) ∆I
L
The ripple current is determined by:
The ripple current is only dependent on inductance
value and is independent of input and output voltage.
See the Ripple Current vs. V
BATT
graph in the Typical
Operating Characteristics.
See Table 4 for suggestions about inductor selection.
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (I
RMS ) imposed by the switching currents.
Ceramic capacitors are preferred due to their resilience
to power-up surge currents:
at 50% duty cycle.
The input capacitors should be sized so that the tem-
perature rise due to ripple current in continuous con-
duction does not exceed about 10°C. The maximum
ripple current occurs at 50% duty factor or V
SRC = 2 x
V BATT , which equates to 0.5 x I CHG . If the application
of interest does not achieve the maximum value, size
the input capacitors according to the worst-case condi-
tions. See Table 5 for suggestions about input capaci-
tor selection.
I I
V V V
V
I
RMS CHG
BATT SRC BATT
SRC
CHG =
−
( ) 







=
2
∆I
k
L
L
OFF =
L
k
x I
OFF
CHG
.
=
0 4
Table 4. Recommended Inductors
APPLICATION (A) INDUCTOR SIZE (mm) L (µH) I SAT (A) R L (m Ω Ω Ω Ω)
2.5 CDRH6D38 8.3 x 8.3 x 3 3.3 3.5 20
2.5 CDRH8D28 7 x 7 x 4 4.7 3.4 24.7
3.5 CDRH8D38 8.3 x 8.3 x 4 3.5 4.4 24
Table 5. Recommended Input Capacitors
APPLICATION (A) INPUT CAPACITOR CAPACITANCE( µF) VOLTS (V) RMS AT 10°C (A)
< 3 GMK316F47S2G 4.7 35 1.8
< 4 GMK325F106ZH 4.7 35 2.4
< 4 TMK325BJ475MN 10 25 2.5