Low-cost multichemistry battery chargers, Component selection – Rainbow Electronics MAX8724 User Manual
Page 24
MAX1908/MAX8724
Choose crossover frequency f
CO_CS
to be 1/5th the
MAX1908/MAX8724 switching frequency:
Solving for C
CS
, C
CS
= 2nF.
To be conservative, set C
CS
= 10nF, which sets the
crossover frequency at:
The compensation pole, f
P_CS
is set at:
Component Selection
Table 2 lists the recommended components and refers
to the circuit of Figure 2. The following sections
describe how to select these components.
Inductor Selection
Inductor L1 provides power to the battery while it is
being charged. It must have a saturation current of at
least the charge current (I
CHG
), plus 1/2 the current rip-
ple I
RIPPLE
:
I
SAT
= I
CHG
+ (1/2) I
RIPPLE
Ripple current varies according to the equation:
I
RIPPLE
= (V
BATT
) × t
OFF
/ L
where:
t
OFF
= 2.5µs × (V
DCIN
– V
BATT
) / V
DCIN
V
BATT
< 0.88 × V
DCIN
or:
t
OFF
= 0.3µs
V
BATT
> 0.88 × V
DCIN
Figure 11 illustrates the variation of ripple current vs.
battery voltage when charging at 3A with a fixed input
voltage of 19V.
Higher inductor values decrease the ripple current.
Smaller inductor values require higher saturation cur-
rent capabilities and degrade efficiency. Designs for
ripple current, I
RIPPLE
= 0.3 × I
CHG
usually result in a
good balance between inductor size and efficiency.
Input Capacitor
Input capacitor C1 must be able to handle the input
ripple current. At high charging currents, the DC-DC
converter operates in continuous conduction. In this
case, the ripple current of the input capacitor can be
approximated by the following equation:
where:
I
C1
= input capacitor ripple current.
D = DC-DC converter duty ratio.
I
CHG
= battery-charging current.
Input capacitor C1 must be sized to handle the maxi-
mum ripple current that occurs during continuous con-
duction. The maximum input ripple current occurs at
50% duty cycle; thus, the worst-case input ripple cur-
rent is 0.5 × I
CHG
. If the input-to-output voltage ratio is
such that the DC-DC converter does not operate at a
50% duty cycle, then the worst-case capacitor current
occurs where the duty cycle is nearest 50%.
The input capacitor ESR times the input ripple current
sets the ripple voltage at the input, and should not
exceed 0.5V ripple. Choose the ESR of C1 according to:
The input capacitor size should allow minimal output
voltage sag at the highest switching frequency:
I
C
dV
dt
C1
2
1
=
ESR
V
I
C
C
1
1
0 5
<
.
I
I
D
D
C
CHG
1
2
=
−
f
R
C
Hz
P CS
OGMS
CS
_
.
=
×
=
1
2
0 0016
π
f
GMS
nF
kHz
CO CS
_
=
=
2 10
16
π
f
GMS
C
kHz
CO CS
CS
_
=
=
2
80
π
Low-Cost Multichemistry Battery Chargers
24
______________________________________________________________________________________
RIPPLE CURRENT vs. V
BATT
V
BATT
(V)
RIPPLE CURRENT (A)
14
13
12
11
10
9
0.5
1.0
1.5
0
8
15 16 17 18
V
DCIN
= 19V
VCTL = ICTL = LDO
4 CELLS
3 CELLS
Figure 11. MAX1908 Ripple Current vs. Battery Voltage