MC33680 Datasheet, PDF (13/16 Pages) ON Semiconductor – Dual DC-DC Regulator for Electronic Organizer

MC33680
DETAILED OPERATING DESCRIPTION (Cont’d)
Current Limit for Both regulators
+ ǒ ) Ǔ From Figure 18 and Figure 21, sense devices (senseFET
or senseBJT) are applied to sample coil current as the
low–side switch is ON. With that sample current flowing
through a sense resistor, sense–voltage is developed.
+ ǒ ) Ǔ _{Threshold detector (COMP2 in both Figures) detects}
VLOBAThigh 1.1
VLOBATlow 0.85
1
RLBa
RLBb
(V)
1
RLBa
RLBb
(V)
whether the sense–voltage is higher than preset level. If it
happens, detector output reset the flip–flop to switch OFF
low–side switch, and the switch can only be ON as next
cycle starts.
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.
Power–Good Signal
If LIBATCL is LOW, the switch, which is then controlled
During the startup period (see Figure 2), the internal
startup circuitry is enabled to pump up VMAIN to a certain
voltage level, which is the user–defined VMAIN output
level minus an offset of 0.15V. The internal Power–Good
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
signal is then enabled to activate the main regulator and ^{microprocessor through LIBATON. The truth table is shown}
conditionally the auxiliary regulator. Meanwhile, the startup
circuitry will be shut down. The Power–Good signal block
also starts to charge up the external capacitor tied from Pin
PDELAY to ground with precise constant current. As the Pin
PDELAY’s voltage reaches an internal set threshold, Pin
PORB will go HIGH to awake the microprocessor. This
+ ^{ǒ Ǔ delay is stated as follows;}
TPOR
1.22
0.5
Cpor
RIref (S)
From Figure 3, if, by any chance, VMAIN is dropped
below the user–defined VMAIN output level minus 0.5V,
PORB will go LOW to indicate the OUTPUT LOW
situation. And, the IC will continue to function until the
in Figure 22.
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 capacitors in parallel
with RMAINb, RAUXa and RAUXb 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.
VMAIN is dropped below 2V.
Bypass Capacitors
Low–Battery–Detect
The Low–Battery–Detect block is actually a voltage
comparator. Pin LOWBAT is LOW, if the voltage of external
Pin LOWBATSEN is lower than 0.85V. The IC will neglect
this warning signal. Pin LOWBAT will become HIGH, if the
voltage of external Pin LOWBATSEN is recovered to more
than 1.1V. From Figure 1, with external resistors RLBa and
If the metal lead 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
should be placed closely to the coil, and connected from
VBAT to ground. This reduces AC component of coil current
passing through the long metal lead, thus minimizing that
portion of power loss.
RLBb, thresholds of Low–Battery–Detect can be adjusted
based on the equations below.
LIBATCL
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ₀
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ ₁
ÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁÁ 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 22. Lithium Battery Backup Control Truth Table
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