Datasheet

L6924D Operation description
Doc ID 11908 Rev 9 15/38
Where,
I
LIM
= current limit of the wall adapter, and R
DS(on)
= resistance of the power MOSFET.
The difference between the set charge current and the adapter limit should be high enough
to minimize the R
DS(on)
value (and the power dissipation). This makes the control loop
completely unbalanced and the power element is fully turned on.
Figure 8 shows the R
DS(on)
values for different output voltage and charging currents for an
adapter current limit of 500 mA.
Figure 8. R
DS(on)
curves vs charging current and output voltage
Neglecting the voltage drop across the charger (ΔV
MOS
) when the device operates in this
condition, its input voltage is equal to the battery one, and so a very low operating input
voltage (down to 2.5 V) is required. The power dissipated by the device during this phase is:
Equation 4
When the battery voltage approaches the final value, the charger gets back the control of
the current, reducing it. Due to this, the upstream adapter exits the current limit condition
and its output goes up to the regulated voltage V
ADP
. This is the worst case in power
dissipation:
Equation 5
In conclusion, the advantage of the linear charging approach is that the designer has the
direct control of the charge current, and consequently the application can be very simple.
The drawback is the high power dissipation.
The advantage of the Quasi-Pulse charging method is that the power dissipated is
dramatically reduced. The drawback is that a dedicated upstream adapter is required.
2
)( LIMonDSCH
IRP ×=
LIMBATADPDIS
IVVP
×
−
=
)(