Datasheet

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x1 Operating
Area
x1.5
Operating
Area
V
I
V
I
V
O
V
A
V
F
V
DX
CP WLEDDriver
V
B
V
C
V
HYS
LED CURRENT SINKS (DM_, DS_)
TPS60251
SLVS767C – APRIL 2007 – REVISED NOVEMBER 2007
Figure 13. Input Voltage Hysteresis Between × 1 and × 1.5 Mode
As shown in Figure 13 , there is input voltage 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 TPS60251 operates 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 WLED driver provides a typical
280mV hysteresis voltage (V
HYS
) that changes based on LED current, to prevent oscillating between modes.
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 TPS60251 switches to 1.5 × mode when the input voltage is below V
B
and remains 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
= 100 mV
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.
The TPS60251 has constant current sinks which drive seven individual LED current paths. Each current sink
regulates the LED current to a constant value determined by the I
2
C interface. The internal register addressing
allows the LED main channels DM1~DM5 to be controlled independently from the LED sub channels DS1~DS2.
The maximum current is programmable by the user (see the Setting the LED Current section). All the LED
channels sink up to 25mA of current except DM5 which has an 80-mA maximum current when configured as an
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