Datasheet
x1 Operating
Area
x1.5
Operating
Area
V
I
V
I
V
O
V
A
V
F
V
DX
CP WLEDDriver
V
B
V
C
V
HYS
TPS60250
TPS60252
SLVS769C – APRIL 2007 – REVISED APRIL 2008 ............................................................................................................................................................
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Figure 13. Input Voltage Hysteresis Between × 1 and × 1.5 Mode
As shown in Figure 13 , there is input hysteresis voltage between 1 × and 1.5 × mode to ensure stable operation
during mode transition. For the 1 cell Li-Ion battery input voltage range, the TPS6025/2 operate in 1 × mode when
a fully charged battery is installed. Once the battery voltage drops below the V
B
level, which is the mode
transition voltage from 1 × to 1.5 × , the WLED driver operates in 1.5 × mode. Once in 1.5 × mode, the battery
voltage must rise to the V
C
level in order to transition from 1.5 × to 1 × . This hysteresis ensures stable operation
when there is some input voltage fluctuation at the 1 × /1.5 × mode transition.
The transition voltage, V
B
, depends on V
DX
(the mode transition threshold voltage), V
F
(WLED forward voltage
drop) and V
A
(the drop out voltage of the charge pump stage) and is calculated as follows:
V
B
= V
A
+ V
F
+ V
DX
V
A
= R
OUT1X
× I
LEDTOTAL
Where R
OUT1X
is the 1 × mode output impedance of the IC. See the Electrical Characteristics table for output
impedance specifications.
The TPS60250/2 switch up to 1.5 × mode when the input voltage is below V
B
and remain in 1.5 × mode as long as
the input is lower than V
C
. 1.5 × Mode is exited when the input voltage rises above V
C
. V
C
is calculated as:
V
C
= V
F
+ 550 mV
The input voltage mode transition hysteresis voltage (V
HYS
) between 1 × and 1.5 × is calculated using the following
equation.
V
HYS
= V
C
– V
B
= 550 mV – V
DX
– V
A
, where V
DX
= 100mV
Note that V
A
is the key factor in determining V
HYS
and is dependant on the 1 × mode charge pump output
impedance and WLED current.
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