MAX8724ETI-T Maxim Integrated, MAX8724ETI-T Datasheet - Page 26

• Manufacturer Part Number: MAX8724ETI-T
• Description: Battery Management
• Manufacturer: Maxim Integrated
• Series: MAX1908, MAX8724, MAX8765, MAX8765Ar

Specifications of MAX8724ETI-T

• Battery Type: Li-Ion, Li-Polymer, NiCd, NiMH, Lead Acid, Universal
• Operating Supply Voltage: 2.5 V to 3.6 V
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Charge Safety Timers: Yes
• Mounting Style: SMD/SMT
• Operating Supply Current: 400 uA
• Product Type: Charge Management
• Temperature Monitoring: No
• Uvlo Start Threshold: 7 V
• Uvlo Stop Threshold: 7.85 V

MAX1908/MAX8724/MAX8765/MAX8765A
26
Low-Cost Multichemistry Battery Chargers
Choose crossover frequency f
MAX1908/MAX8724/MAX8765/MAX8765A 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:
Figure 11. Ripple Current vs. Battery Voltage
RIPPLE
:
f
P CS
1.5
1.0
0.5
CS
_
0
f
I
CO CS
I
SAT
f
8
, C
RIPPLE
CO CS
3 CELLS
V
VCTL = ICTL = LDO
DCIN
=
_
9
CS
_
= I
2
= 19V
10
π
Component Selection
= 2nF.
RIPPLE CURRENT vs.
CHG
R
= (V
BATTERY VOLTAGE
=
11
=
OGMS
2
P_CS
2 10
GMS
CHG
π
12
GMS
BATT
π
+ (1/2) I
V
C
1
CS
BATT
CS
13
×
nF
), plus 1/2 the current rip-
is set at:
(V)
= 10nF, which sets the
) × t
C
Inductor Selection
14
CO_CS
=
CS
=
80
RIPPLE
15 16 17 18
16
OFF
kHz
=
4 CELLS
kHz
0 0016
/L
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
CHG
= 2.5µs × (V
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
CHG
< 0.88 × V
> 0.88 × V
C
=
1
DCIN
= 0.3µs
C
<
1
0 5
D D
dV
I
dt
.
C
– V
−
1
CHG
V
DCIN
DCIN
BATT
2
Input Capacitor
usually result in a
)/V
DCIN
Maxim Integrated