MAX8731 Maxim Integrated Products, MAX8731 Datasheet - Page 29

• Manufacturer Part Number: MAX8731
• Description: SMBus Level 2 Battery Charger
• Manufacturer: Maxim Integrated Products

The input capacitor must meet the ripple current
requirement (I
Nontantalum chemistries (ceramic, aluminum, or OS-
CON) are preferred due to their resilience to power-up
surge currents:
The input capacitors should be sized so that the temper-
ature rise due to ripple current in continuous conduction
does not exceed approximately 10°C. The maximum rip-
ple current occurs at 50% duty factor or V
V
interest does not achieve the maximum value, size the
input capacitors according to the worst-case conditions.
The output capacitor absorbs the inductor ripple current
and must tolerate the surge current delivered from the
battery when it is initially plugged into the charger. As
such, both capacitance and ESR are important parame-
ters in specifying the output capacitor as a filter and to
ensure stability of the DC-DC converter (see the
Compensation section). Beyond the stability require-
ments, it is often sufficient to make sure that the output
capacitor’s ESR is much lower than the battery’s ESR.
Either tantalum or ceramic capacitors can be used on the
output. Ceramic devices are preferable because of their
good voltage ratings and resilience to surge currents.
Smart-battery systems have evolved since the concep-
tion of the smart-battery system (SBS) specifications.
Originally, such systems consisted of a smart battery
and smart-battery charger that were compatible with the
SBS specifications and communicated directly with one
another using SMBus protocols. Modern systems still
employ the original commands and protocols, but often
use a keyboard controller or similar digital intelligence to
mediate the communication between the battery and the
charger (Figure 13). This arrangement permits consider-
able freedom in the implementation of charging algo-
rithms at the expense of standardization. Algorithms can
vary from the simple detection of the battery with a fixed
set of instructions for charging the battery to highly com-
plex programs that can accommodate multiple battery
BATT
I
, which equates to 0.5 x I
RMS
=
Smart-Battery System Background
I
CHG
RMS
Applications Information
SMBus Level 2 Battery Charger with
⎛
⎜
⎜
⎝
) imposed by the switching currents.
______________________________________________________________________________________
V
Output Capacitor Selection
BATT
Input Capacitor Selection
(
V
V
DCIN
DCIN
CHG
−
V
. If the application of
BATT
Information
)
⎞
⎟
⎟
⎠
DCIN
= 2 x
configurations and chemistries. Microcontroller pro-
grams can perform frequent tests on the battery’s state
of charge and dynamically change the voltage and cur-
rent applied to enhance safety. Multiple batteries can
also be utilized with a selector that is programmable over
the SMBus.
The input current limit should be set based on the cur-
rent capability of the AC adapter and the tolerance of
the input current limit. The upper limit of the input cur-
rent threshold should never exceed the adapter’s mini-
mum available output current. For example, if the
adapter’s output current rating is 5A ±10%, the input
current limit should be selected so that its upper limit is
less than 5A × 0.9 = 4.5A. Since the input current-limit
accuracy of the MAX8731 is ±3%, the typical value of
the input current limit should be set at 4.5A / 1.03 ≈
4.36A. The lower limit for input current must also be
considered. For chargers at the low end of the spec,
the input current limit for this example could be 4.36A ×
0.95, or approximately 4.14A.
Bypass DCIN with a 1µF ceramic to ground (Figure 1).
D1 protects the MAX8731 when the DC power source
input is reversed. Bypass V
and REF as shown in Figure 1.
Figure 13. Typical Smart-Battery System
SMART-BATTERY
POWER-SOURCE
CONVERTER
Remote Sense
(ADAPTER)
CHARGER/
SELECTOR
AC-TO-DC
MAX8731
CONTROL
SIGNALS
BATTERY
SMBus
FOR
Setting Input Current Limit
(KEYBOARD CONTROLLER)
SYSTEM HOST
Layout and Bypassing
DD
BATT+
BATT-
, DCIN, LDO, VCC, DAC,
SUPPLIES
BATTERY
CONTROL
SYSTEM
SIGNALS
BATTERY
POWER
SMART
SMBus
FOR
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