Datasheet

Charging Control
TPS65563A
www.ti.com ......................................................................................................................................................... SLVS858A – JULY 2008 – REVISED AUGUST 2008
Figure 5 shows a timing diagram at beginning/ending. The TPS65563A provides three comparators to control the
charging operation. U1 is the V
FULL
comparator to detect the charge completion, U2 is the V
ZERO
comparator to
detect the turn-on time of the power SW, and U3 is the I
PEAK
comparator to detect the turn-off time of the power
SW.
Figure 5. Beginning/Ending Timing
While the power SW is ON, the I
PEAK
comparator (U3) monitors current flow through the power SW from SW to
GND. When the current at SW (I
SW
) exceeds the threshold defined by the voltage of the I_PEAK pin (I
PEAK
), the
power SW turns OFF.
After the power SW turns OFF, the spike voltage occurs immediately because of leakage inductance at the
primary side. It might cause the power SW to break. To avoid this, the leakage inductance should be reduced as
much as possible.
When the power SW is OFF, the magnetic energy in the transformer starts discharging from the primary side to
the secondary side. During this discharge, the V
ZERO
comparator (U2) monitors the kickback voltage at the
primary side to compare it with the VBAT voltage. The kickback voltage increases rapidly until the diode placed
at secondary side turns ON. The diode turns ON when the voltage of secondary side of the transformer reaches
more than the voltage of the output capacitor. After the diode turns ON, the kickback voltage is almost stable
until the magnetic energy at the primary side discharges completely.
After the discharge stops, the small amount of energy left in the transformer is released via parasitic paths, and
the kickback voltage reaches almost zero voltage. During this period, U2 makes the power SW turn ON when
(V
SW
– VBAT) drops from V
ZERO
.
The V
FULL
comparator (U1) also monitors the kickback voltage. When V
SW
– VBAT exceeds V
FULL
, the
TPS65563A stops the charging operation. After detection, XFULL goes to low level to indicate charge
completion. After charge completion, the TPS65563A immediately goes into disable mode with the internal ENA
automatically turning to a low level. The purpose is to save the consumption power.
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