MAX8724 Maxim Integrated Products, MAX8724 Datasheet - Page 26

• Manufacturer Part Number: MAX8724
• Description: (MAX1908 - MAX87xx) Low-Cost Multichemistry Battery Chargers
• Manufacturer: Maxim Integrated Products

Choose crossover frequency f
MAX1908/MAX8724/MAX8765 switching frequency:
Solving for C
To be conservative, set C
crossover frequency at:
The compensation pole, f
Table 3 lists the recommended components and refers
to the circuit of Figure 2. The following sections
describe how to select these components.
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
ple I
Ripple current varies according to the equation:
Low-Cost Multichemistry Battery Chargers
Figure 11. Ripple Current vs. Battery Voltage
26
RIPPLE
______________________________________________________________________________________
:
1.5
1.0
0.5
f
P CS
0
CS
_
8
I
f
I
RIPPLE
CO CS
f
SAT
, C
CO CS
3 CELLS
V
VCTL = ICTL = LDO
DCIN
9
=
_
CS
_
= I
2
= 19V
10
π
RIPPLE CURRENT vs.
Component Selection
= 2nF.
CHG
= (V
BATTERY VOLTAGE
R
11
=
=
OGMS
2
P_CS
2 10
12
GMS
CHG
π
BATT
V
GMS
π
+ (1/2) I
C
1
BATT
CS
13
CS
×
nF
), plus 1/2 the current rip-
(V)
is set at:
) × t
= 10nF, which sets the
C
Inductor Selection
14
CO_CS
=
CS
=
80
15 16 17 18
RIPPLE
OFF
16
kHz
=
4 CELLS
kHz
/ L
0 0016
to be 1/5th the
.
Hz
where:
or:
Figure 11 illustrates the variation of ripple current vs.
battery voltage when charging at 3A with a fixed 19V
input voltage.
Higher inductor values decrease the ripple current.
Smaller inductor values require higher saturation cur-
rent capabilities and degrade efficiency. Designs for
ripple current, I
good balance between inductor size and efficiency.
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
D = DC-DC converter duty ratio.
I
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
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:
C1
CHG
= input capacitor ripple current.
= battery-charging current.
t
OFF
= 2.5µs × (V
CHG
RIPPLE
V
V
I
. If the input-to-output voltage ratio is
BATT
BATT
C
1
ESR
=
t
I
OFF
C1
2
I
= 0.3 × I
< 0.88 × V
> 0.88 × V
CHG
C
DCIN
=
1
= 0.3µs
C
<
1
0 5
D D
I
dV
dt
– V
.
C
−
1
CHG
V
BATT
DCIN
DCIN
2
Input Capacitor
usually result in a
) / V
DCIN