MAX1535A Datasheet, PDF (35/39 Pages) Maxim Integrated Products – Highly Integrated Level 2 SMBus Battery Charger

Highly Integrated Level 2 SMBus
Battery Charger
1.5
3 CELLS (12.6V)
4 CELLS (16.8V)
1.0
0.5
L = 10µH
V_DCIN = 19V
CHARGE CURRENT = 3A
0
8 9 10 11 12 13 14 15 16 17 18
V_BATT (V)
Figure 16. Ripple Current vs. Battery Voltage
Input Capacitor Selection
The input capacitor must meet the ripple current
requirement (I_RMS) imposed by the switching currents.
Nontantalum chemistries (ceramic, aluminum, or OS-
CON) are preferred due to their resilience to power-up
surge currents:

I_RMS = I_{CHG}
V_BATT
(V_DCIN −
V_DCIN
V_BATT
)


The input capacitors should be sized so that the tem-
perature rise due to ripple current in continuous con-
duction does not exceed approximately 10°C. The
maximum ripple current occurs at 50% duty factor or
V_DCIN = 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 conditions.
Output Capacitor Selection
The output capacitor absorbs the inductor ripple cur-
rent 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
parameters in specifying the output capacitor as a filter
and to ensure the stability of the DC-to-DC converter
(see the Compensation section.) Beyond the stability
requirements, 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.
Applications Information
Smart-Battery System Background
Information
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
each other using SMBus protocols. Modern systems
still employ the original commands and protocols, but
often use a keyboard controller or similar digital intelli-
gence to mediate the communication between the bat-
tery and the charger (Figure 17). This arrangement
permits considerable freedom in the implementation of
charging algorithms 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 complex programs that can accommo-
date multiple battery configurations and chemistries.
Microcontroller programs can perform frequent tests on
the battery’s state of charge and dynamically change
the voltage and current applied to enhance safety.
Multiple batteries can also be utilized with a selector
that is programmable over the SMBus.
Batteries that use SMBus fuel gauges must sometimes
perform a conditioning cycle to calibrate the fuel
gauge’s reference data for empty and full capacity.
This cycle consists of isolating the battery from the
charger and discharging it through the system load.
When the battery reaches 100% depth of discharge, it
is then recharged. The circuit in Figure 1 is capable of
implementing this feature under software control. To uti-
lize the conditioning function, the configuration of the
PDS switch must be changed to source-connected
FETs to prevent the AC adapter from supplying current
to the system through the MOSFET’s body diode. The
SRC pin must be connected to the common source
node of the back-to-back FETs to properly drive the
MOSFETs.
It is essential to alert the user that the system is perform-
ing a conditioning cycle. If the user terminates the cycle
prematurely, the battery may be discharged even
though the system was running off an AC adapter for a
substantial period of time. If the AC adapter is in fact
removed during conditioning, the MAX1535A keeps the
PDL switch on and the charger remains off as it would in
normal operation. If the battery is removed during condi-
tioning mode, the PDS switch is turned back on and the
system is powered from the AC adapter.
______________________________________________________________________________________ 35