MC34280 Datasheet, PDF (16/20 Pages) ON Semiconductor – Power Supply & Management IC for Handheld Electronic Products

MC34280
DETAILED OPERATING DESCRIPTION (Cont’d)
Lithium–Battery backup
The backup conduction path which is provided by an
internal power switch (typ. 13 Ohm) can be controlled by
internal logic or microprocessor.
If LIBATCL is LOW, the switch, which is then controlled
by internal logic, is ON when the battery is removed and
VMAIN is dropped below LIBATIN by more than 100mV,
and returns OFF when the battery is plugged back in.
If LIBATCL is HIGH, the switch is controlled by
microprocessor through LIBATON. The truth table is shown
in Figure 26.
Efficiency and Output Ripple
For both regulators, when large values are used for
feedback resistors (> 50kOhm), stray capacitance of pin 1
(VMAINFB) and pin 20 (VAUXFBN) can add ”lag” to the
feedback response, destabilizing the regulator and creating
a larger ripple at the output. From Figure 1, ripple of Main
and AUX regulator can be reduced by CMAINb, CAUXa
and CAUXb ranging from 100pF to 100nF respectively.
Reducing the ripple is also with improving efficiency,
system designers are recommended to do experiments on
capacitance values based on the PCB design.
Bypass Capacitors
If the metal leads from battery to coils are long, its stray
resistance can put additional power loss to the system as AC
current is being conducted. In that case, bypass capacitors
(CMAINbp and CAUXbp of Figure 1) are recommended to
remove AC components of coil currents to minimize that
power loss to optimize efficiency.
LIBATCL
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ₀
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 1
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 1
LIBATON
X
0
1
Action
The switch is ON when the battery is removed and VMAIN is dropped below LIBATIN
by more than 100mV; The switch is OFF when the battery is plugged in.
The switch is OFF
The switch is ON
Figure 26. Lithium Battery Backup Control Truth Table
http://onsemi.com
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