SLEA004
Extending Battery Life With the TPS61040 White Light LED Driver
3
Figure 2 shows a TPS61040 application that drives four LEDs and includes the capability to dim
their light output. The difference between the two circuits is the addition of Vbias and the Rbias
resistor to Figure 2. The following equations calculate the correct resistor values for the circuit.
R
4
V
cs
I
out_max
:=
R
2
R
bias
V
adj_max
V
fb
−
(
)
V
cs
⋅
V
fb
V
adj_min
V
adj_max
−
(
)
⋅
+
V
fb
V
b
−
(
)
V
cs
⋅
⋅
:=
R
3
V
fb
V
cs
−
(
)
R
bias
⋅
R
2
⋅
R
2
V
b
⋅
R
2
V
fb
⋅
−
R
bias
V
adj_min
⋅
R
bias
V
fb
⋅
−
+
(
)
:=
Where:
Vcs is the desired maximum voltage across the current sense resistor.
Iout_max is the maximum LED current.
Rbias is the chosen value for the additional bias resistor.
Vadj_min is the minimum voltage used to adjust the LED current.
Vadj_max is the maximum voltage used to adjust the LED current.
Vfb is the reference voltage of the TPS61040 (1.233 V).
Vb is the bias voltage.
Figure 2.
Improved White Light LED Driver Extends Battery Life
